Preparation method of aluminum-doped and high-hydrated chromite single crystal under high temperature and high pressure

Abstract

The application discloses a preparation method of aluminum-doped and high-hydrated chromite monocrystal under high temperature and high pressure, and the method comprises the following steps: preparing a cylindrical chromite sample by taking solid triangular ferrous carbonate crystals, solid alkaline chromium acetate crystal powder, solid aluminum distearate powder, solid oxalic acid powder, solid alpha-phase goethite powder, solid aluminum hydroxide powder and liquid dilute nitric acid as starting raw materials; preparing two pieces of water source pieces by taking the alpha-phase goethite powder and the aluminum hydroxide powder in a weight ratio of 4:1; placing the two pieces of water source pieces at two ends of the cylindrical chromite sample and putting the sample into a double-capsule structure sample bin to perform a high-temperature and high-pressure reaction to obtain the chromite monocrystal; and the method solves the technical blank of the preparation of aluminum-doped and high-hydrated chromite large-grain monocrystal under high temperature and high pressure, so that the experimental sample of the aluminum-doped and high-hydrated chromite large-grain monocrystal can be obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of mineral single crystal sample synthesis, and in particular relates to a method for preparing aluminum-doped and high-water-content chromite single crystals under high temperature and high pressure. Background Art

[0002] Chromite, a key end-member of the chromite subgroup of the spinel family of minerals, has a chemical formula of FeCr2O4 and is a significant iron- and chromium-rich oxide mineral. The mineralogy of chromite, with its oxide chemical composition percentages, can be expressed as: FeO / (FeO + Cr2O3) = 32.1% and Cr2O3 / (FeO + Cr2O3) = 67.9%. Due to its high chromium content, chromite is the primary raw material for the industrial production of chromium. Chromite is the most typical oxide mineral with a normal spinel structure found in nature. In its unit cell, the cubic closest packing fraction of chromite, which exhibits the distinct physical and chemical characteristics of a normal spinel structure, is zero.

[0003] According to the carbon content in the chromite, geoscientists further divide the chromite into: high-carbon chromite (carbon content: 4-8%), medium-carbon chromite (carbon content: 0.5-4%), low-carbon chromite (carbon content: 0.15-0.5%), micro-carbon chromite (carbon content: 0.15-0.06%), and ultra-micro-carbon chromite (carbon content: <0.03%) and other types of chromite. Generally, the chromite monomineral exposed on the earth's surface is mostly in the form of massive or granular aggregates, which is extremely similar to magnetite in the magnetite subfamily of the spinel mineral group. The isometric crystal aggregate of chromite monocrystal is the product of magmatic action, which occurs in super-magmatic rocks and is associated with silicates such as olivine, and is also exposed in placer deposits. Existing regional mineral geological data shows that nearly 60 chromite mines have been identified in China, and the chromite resources are distributed in 13 provinces, municipalities and autonomous regions in China, among which Tibet has the most, with a reserve of more than 400 tons, accounting for 40% of the country. Chromite deposits are distributed in Inner Mongolia, Xinjiang, Gansu, Hebei, Jilin, Hubei, Shaanxi, Shanxi, Sichuan, Yunnan, etc. They are mainly concentrated in the southwest, northwest and north of China. Among them, Luobusha in Tibet, Daoduerji in Gansu, Sartohai in Xinjiang and Hegenshan in Inner Mongolia are the four largest medium-sized chromite deposit distribution areas in China. Based on the existing geological data on ore genesis and comprehensive utilization of mineral resources, the four medium-sized chromite deposits discovered in China have the following characteristics: (1) the chromite deposit ore body is not large in size and is scattered in distribution; (2) the chromite mineral resources are unevenly distributed, and the resource development and utilization conditions are poor; (3) the available chromite lean ore reserves are about 4.993 million tons, accounting for 46.3%; the available chromite lean ore reserves are about 5.786 million tons, accounting for 53.7%, so the chromite lean ore reserves and rich ore reserves are roughly equal; (3) the available chromite mineral resources are almost exhausted, and the small-scale and easy-to-mine chromite mineral resources are almost exhausted; (4) the main ore-forming era and main genetic type of the chromite mineral resources discovered in China today are mainly late magmatic deposits, and the chromite deposit genetic type is relatively single. In the world, large chromite deposits are mainly found in the Transvaal region of South Africa, Zimbabwe, Guilo, Russia's Svidlovsk region, Cuba and other countries and regions.

[0004] In the chromite crystal structure, the non-ferrous light metal aluminum element located in the third period and group IIIA can easily occupy the octahedral position, thereby forming an isomorphic substitution of the trivalent cation at the B position. Since in chromite, the chromium element in the lattice position and the doped aluminum element have the same positive trivalent valence, this isomorphic substitution belongs to an equivalent isomorphic substitution. In the earth's crust, aluminum is the most abundant metal element, with a content of up to 8.3%. Aluminum-containing minerals widely exposed on the earth's surface, such as cryolite, feldspar, mica, kaolinite, bauxite, alunite, etc., are all important aluminum-containing ore minerals. The aluminum oxide mineral, corundum, has three main forms of isomorphic variants, namely α-Al2O3, β-Al2O3 and γ-Al2O3. l2 O3, its hardness is second only to diamond. In nature, corundum is typically formed in high-temperature, aluminum-rich, silicon-poor igneous rocks and pegmatites. It is a key product of endogenous geological processes, coexisting with oxide minerals such as spinel and feldspar, as well as silicate minerals. Under intermediate to advanced metamorphic conditions, corundum produced in gneiss coexists with magnetite, muscovite, and skarn. Corundum produced in the contact zone between igneous rocks and limestone coexists with magnetite, calcite, and epidote, representing the desiliconization of igneous rocks. Corundum, with its acid resistance, high hardness, and high melting point, is widely used in the industrial production and manufacturing of abrasives, refractories, and mechanical bearings. Corundum containing trace elements of trivalent cation chromium, resulting in a reddish color, is called ruby; corundum containing trace elements of divalent and trivalent iron and tetravalent titanium is called sapphire.

[0005] Chromite, with its spinel structure, contains no water molecules or hydroxyl groups in its molecular structure, exhibiting apparent nominally anhydrous properties. However, previous experimental results on the water solubility of spinel under high-temperature and high-pressure conditions, as measured by infrared spectroscopy, indicate that spinel can dissolve water in amounts up to hundreds of ppm. Water is one of the most important volatile components in all major layers of the Earth's interior, particularly in the mantle transition zone, extending from 410 km to 660 km (corresponding to pressures and temperatures of 16.0-23.0 GPa and 1450-1800°C). Previous experimental studies of physical properties and spectroscopic data of minerals and rocks under high-temperature and high-pressure conditions, such as electrical conductivity, Brillouin scattering elastic wave velocity, thermal diffusivity, thermal conductivity, and vacuum Fourier transform infrared spectroscopy, have demonstrated that trace amounts of water in nominally anhydrous minerals can enhance their physical and spectroscopic properties by several orders of magnitude, significantly impacting their physical properties. A review of the methods used to synthesize chromite in laboratory materials science both domestically and internationally reveals that the main methods employed include high-temperature solid-phase sintering, polyvinyl acetate alcoholysis, carbonate co-precipitation, high-pressure powder hydrothermal method, metal alkoxide sol-gel method, microwave-activated solid-phase reaction, and freeze-drying. Most of these existing synthesis techniques employ simple solution chemical reactions or direct physical grinding of sample powder particles followed by high-temperature sintering, making them more suitable for producing nanosized chromite crystals. Since high-temperature and high-pressure experimental geoscience research typically requires single crystal mineral samples with micron-sized or larger particle sizes, it is clear that nanosized chromite samples obtained through previous material synthesis have failed to meet the required minimum particle size, and an effective synthesis method has yet to be developed. Previously, geoscience researchers have often used natural chromite samples instead of synthetic ones to meet the needs of high-temperature and high-pressure experimental geoscience research. However, these natural samples suffer from significant heterogeneity in the distribution of trace element aluminum. Therefore, it is particularly urgent to effectively synthesize a large-particle aluminum-doped and highly hydrated chromite single crystal that meets the needs of various high-temperature and high-pressure laboratory simulations of earth science research, especially the study of the preferred lattice orientation and crystal axis anisotropy of chromite single crystal minerals under high pressure. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for preparing aluminum-doped and high-water-content chromite single crystals under high temperature and high pressure, so as to fill the current technical gap in the preparation of large-particle aluminum-doped and high-water-content chromite single crystals under high temperature and high pressure conditions, and to obtain experimental samples of large-particle aluminum-doped and high-water-content chromite single crystals.

[0007] The technical solution of the present invention is:

[0008] A method for preparing aluminum-doped chromite single crystals under high temperature and high pressure, the method comprising: preparing a cylindrical chromite sample using solid triangular ferrous carbonate crystals, solid basic chromium acetate crystalline powder, solid aluminum distearate powder, solid oxalic acid powder, solid alpha-phase goethite powder, solid aluminum hydroxide powder, and liquid dilute nitric acid as starting materials; preparing two water source pieces from the alpha-phase goethite powder and the aluminum hydroxide powder at a weight ratio of 4:1; placing the two water source pieces at the two ends of the cylindrical chromite sample and placing them in a double-capsule structure sample bin for high temperature and high pressure reaction to obtain the chromite single crystal.

[0009] The solid triangular ferrous carbonate crystals have a purity of >99.99%, the solid basic chromium acetate crystalline powder has a purity of >99.99%, the solid aluminum distearate powder has a purity of >99.99%, the solid oxalic acid powder has a purity of >99.99%, the solid alpha-phase goethite powder has a purity of >99%, the solid aluminum hydroxide powder has a purity of >99%, and the liquid dilute nitric acid has a concentration of 10%.

[0010] The method for preparing the cylindrical chromite sample comprises:

[0011] Step 1. Weigh 60 milliliters of dilute nitric acid with a concentration of 10% and put it all into a notch beaker.

[0012] Step 2. Weigh 5.0 grams of triangular ferrous carbonate crystals and add them to the notch beaker, and place it in a magnetic

[0013] stirring rotor

[0014] Step 3. Cover the notch beaker with a glass surface dish, and at room temperature, rotate it at a speed of 700 revolutions per minute for 72 hours.

[0015] Step 4. According to the stoichiometric ratio of chromite Fe(Cr, Al)2O4, weigh 17.3584 grams of solid basic chromium acetate crystalline powder and 650 milligrams of solid aluminum distearate powder and add them to the notch beaker, respectively.

[0016] Step 4. Cover the notch beaker with a glass surface dish.

[0017] Step 5. Place the notch beaker in a high-temperature magnetic stirring hot plate in a fume hood, and stir it at a speed of 800 revolutions per minute for 48 hours at room temperature.

[0018] Step 6. Weigh 2 grams of solid oxalic acid powder and add it to the notch beaker.

[0019] Step 7. Place the notch beaker in a high-temperature magnetic stirring hot plate in a fume hood, cover it with a glass surface dish, and set the condition parameters of the high-temperature magnetic stirring hot plate to 80°C, and stir it at a speed of 1000 revolutions per minute for 36 hours.

[0020] Step 8: Remove the glass watch glass from the beaker and increase the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the notched beaker is completely evaporated;

[0021] Step 9: Take out all the mixed powders in the notched beaker and place them in a graphite crucible;

[0022] Step 10: The graphite crucible containing the mixture powder was placed in a muffle furnace under normal pressure and high temperature conditions, and the temperature was increased to 1100° C. at a heating rate of 300° C. / hour, and the temperature was kept constant for 5 hours;

[0023] Step 11, cooling the mixed sample powder to room temperature at a cooling rate of 200°C / hour;

[0024] Step 12: Grind the mixed sample powder in a corundum mortar for 1 hour;

[0025] Step 13: cold-press the ground mixed sample powder into three sample discs of Φ10.0 mm × 3.0 mm, and stack the three cold-pressed sample mixtures vertically on top of each other and place them at the bottom of a graphite crucible.

[0026] Step 14: Hang the graphite crucible in the middle of the high-temperature oxygen atmosphere furnace;

[0027] Step 15: Place a stainless steel container filled with secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace;

[0028] Step 16: Connect the top of the high-temperature oxygen atmosphere furnace to an argon inert gas cylinder and carbon monoxide and carbon dioxide cylinders with adjustable ratios;

[0029] Step 17: Open the argon inert gas valve and continue to inflate for 30 minutes. Under the protection of argon inert gas, calcine the sample at a high temperature of 800° C. at a heating rate of 400° C. / hour.

[0030] Step 18: After the temperature inside the furnace reaches 800°C, switch the carbon monoxide gas cylinder and the carbon dioxide gas control valve so that the volume ratio of carbon monoxide and carbon dioxide passing through the sample oxygen atmosphere furnace reaches 4:1;

[0031] Step 19: After the mixed gas flow of carbon monoxide and carbon dioxide in a volume ratio of 4:1 to control the oxygen fugacity in the sample chamber reaches a stable state, the temperature of the sample chamber in the furnace is increased to 1430° C. at a heating rate of 200° C. / hour and the mixture is calcined at a constant temperature for 15 minutes;

[0032] Step 20: After constant temperature roasting at 1430° C. for 15 minutes, the graphite crucible containing the sample is pulled out of the furnace body and directly immersed in a stainless steel container to be quenched into chromite glass;

[0033] Step 21: remove the quenched chromite glass from the graphite crucible, grind it in a corundum mortar, and place the glassy chromite powder in a vacuum drying oven at 200° C. for 12 hours;

[0034] Step 22: cold-press the chromite glass powder on a cold isostatic press to form a cylindrical chromite sample with a diameter of Φ4.0 mm and a height of 4.0 mm.

[0035] The method for preparing two water source sheets by using α-phase goethite powder and aluminum hydroxide powder in a weight ratio of 4:1 is as follows: α-phase goethite powder and aluminum hydroxide powder are cold-pressed into two water source sheets of Φ4.0mm (diameter) × 0.1mm (height) using a tungsten carbide grinding tool of Φ4.0mm (diameter) × 10.0mm in a weight ratio of 4:1.

[0036] The method of placing two water source sheets at both ends of a cylindrical chromite sample and placing the sample in a double-capsule structure sample chamber for high-temperature and high-pressure reaction to obtain a chromite single crystal includes:

[0037] Step 24: Seal the cylindrical chromite sample and two water source tablets in sequence in a double-sleeved experimental sample chamber with a graphite inner casing and a gold-palladium alloy outer casing; place the chromite cylindrical sample in the center of the inner graphite casing; and place the water source tablets at both ends of the inner graphite casing close to the sample.

[0038] Step 25: Place the double-capsule structure sample chamber on a typical 6-8 type multi-faceted large cavity high-temperature and high-pressure equipment in the laboratory, Kawai-1000t. Set the pressure and temperature rise rates to 0.5 GPa / hour and 10°C / minute, respectively. Raise the pressure and temperature to 3.0 GPa and 1100°C, respectively, and perform hot pressing sintering. The reaction time is 72 hours at constant temperature and pressure.

[0039] Step 26: After maintaining constant temperature and pressure at 3.0 GPa and 1100° C. for 72 hours, the temperature in the sample chamber is reduced from 1100° C. to 800° C. at a cooling rate of 3° C. / min and maintained at that temperature for 1 hour; then, the temperature in the sample chamber is reduced from 800° C. to room temperature at a cooling rate of 5° C. / min;

[0040] Step 27: After the temperature in the sample chamber drops to room temperature, the pressure in the sample chamber is reduced from 3.0 GPa to normal pressure at a pressure reduction rate of 0.5 GPa / hour;

[0041] Step 28. After the high-temperature and high-pressure preparation reaction is completed, the sample is removed from the Kawai-1000t typical 6-8 type multi-faceted large-cavity high-temperature and high-pressure equipment. The graphite tube and gold-palladium alloy tube of the double-capsule structure sample chamber that encloses the sample are removed. The cylindrical sample is cut in half using a diamond wire cutter, and the chromite single crystal is selected under an Olympus microscope.

[0042] During high temperature and high pressure reactions, the temperature is calibrated using two sets of tungsten-rhenium thermocouples. Each set of tungsten-rhenium thermocouples is made of two different tungsten-rhenium alloys, with a chemical composition of W. 95％ Re 5％ and W 74％ Re 26％ Each set of tungsten-rhenium thermocouples is symmetrically placed at the upper and lower ends of a double-capsule structure sample chamber composed of a graphite tube and a gold-palladium alloy tube.

[0043] The beneficial effects of the present invention are:

[0044] The present invention organically combines crystallography, mineralogy, crystal structure chemistry, gemology, igneous rock petrology, metamorphic rock petrology, ore deposit geochemistry, mining geology, engineering geology, elemental geochemistry, economic geology, mineral lattice defect science, mineral material processing geology, high-pressure mineral physics, pegmatite petrology, mineralogy, field experimental petrology, advanced geochemistry, regional geology and other related earth science disciplines, and uses a typical laboratory Kawai-1000t 6-8 type multi-faceted large cavity high-temperature and high-pressure equipment to simulate the formation process of aluminum-doped and high-water-content chromite single crystals under high-temperature and high-pressure conditions. The present invention involves the following main chemical reaction equations:

[0045] FeCO3+2HNO3→Fe(NO3)2+CO2+H2O

[0046] 3Fe(NO3)2+2[Cr3(OH)2(OOCCH3)7]→3FeCr2O4+6(NH3·H2O)+4CH4+

[0047] 8CO2+16CO

[0048] FeCr2O4+2[CH3(CH2) 16 COO]3Al→Fe(Cr,Al)2O4+48C2H2+6CO2+57H2

[0049] 2α(FeOOH)→α(Fe2O3)+H2O

[0050] 2α(FeOOH)→6 / (6-x)αFe (2-x / 3) (OH) x O (3-x)+ (6 - 4x) / (6 - x) H2O

[0051] → α (Fe2O3) + 3x / (6 - x) H2O

[0052] 2 Al (OH)3→ Al2O3+ 3 H2O

[0053] The selected initial raw material ferrous carbonate [chemical formula: FeCO3, also known as siderite] is a light gray, glass luster and transparent - translucent triangular crystalline solid material, which has stable chemical properties, is almost insoluble or slightly soluble in water, is easily dissolved in dilute acid solution and foaming. The triangular crystalline ferrous carbonate crystal is selected because of its stable performance and superior characteristics of easy dissolution in dilute acid, and is an excellent raw material for providing iron elements in artificial synthetic chromite. The initial raw material basic chromium acetate [also known as: chromium (III) acetate hydroxide or chromium (III) acetate hydroxide, chemical formula: Cr3(OH)2(OOCCH3)7] is a light gray green to blue solid crystalline powder, which is stable in chemical properties at room temperature, does not decompose, is non-toxic and soluble in water. The basic chromium acetate can be used in the production of high purity metal chromium, some glaze, colored glass and other industries. The basic chromium acetate crystalline powder is selected because of its superior characteristics of easy decomposition and strong chemical reaction activity in dilute acid solution, and is an excellent raw material for providing chromium elements in artificial synthetic chromite. The initial raw material aluminum distearate [also known as: basic aluminum stearate or aluminum distearate, chemical formula: [CH3(CH2) 16 COO]3Al] is a white powder solid material, which is almost insoluble in water, ethanol and diethyl ether and other solvents, is easily dissolved in turpentine, mineral oil, petroleum, kerosene, benzene and other solvents, and is easily decomposed into stearic acid and corresponding aluminum salt in strong acid. The aluminum distearate has a wide application in the fields of heat stabilizer, building waterproof agent, emulsifier, coating, metal antirust agent, thickening agent, lubricant and other industrial production and processing. Because it is dissolved in dilute nitric acid solution, the aluminum distearate is an excellent raw material for providing trace aluminum elements in artificial synthetic chromite. The initial raw material solid α-phase goethite [formula: FeOOH] also belongs to a typical iron-containing hydrated mineral. A previous academic view believes that the α-phase goethite directly generates hematite at a temperature of 270°C by dehydration reaction, while releasing a large amount of water. Another academic view believes that the α-phase goethite generates superstructure hematite [formula: Fe (2-x / 3) (OH) x O (3-x)], while superstructured hematite undergoes a second dehydration reaction at 800°C, producing hematite and simultaneously releasing a large amount of water. The selected starting raw material, aluminum hydroxide [molecular formula: Al(OH)3], is a typical aluminum-rich, white, amorphous, hydrated solid. It undergoes three dehydration reactions at temperatures of 230°C, 300°C, and 500°C, respectively, producing monohydrated aluminum oxide, crystalline aluminum oxide, and corundum, respectively, while simultaneously releasing a large amount of water. Within the high-pressure sample chamber, α-phase goethite and aluminum hydroxide, containing water in a specific ratio, are placed. Under the high-temperature and high-pressure conditions, a dehydration reaction occurs, producing a large amount of water, providing an excellent water source for the synthesis of aluminum-doped and highly hydrated chromite single crystals. Among the chemical reaction products involved in the present invention, the resulting NH3·H2O, CH4, C2H2, CO2, CO, and H2 are all high-temperature volatile substances.

[0054] The present invention requires the synthesis of large-grained single crystals of aluminum-doped and highly hydrated chromite. The synthesized samples contain aluminum-doped chromite single crystals that match the development and comprehensive utilization of aluminum mineral resources, and are widely used in experimental simulation studies of the lithogenesis and mineralization of mineral rocks under high temperature and high pressure conditions. Compared with natural chromite samples exposed in nature, which may be substituted by impurity ions such as magnesium ions, aluminum ions, and nickel ions, the preparation process of the aluminum-doped and highly hydrated chromite single crystals of the present invention is pure in the laboratory environment, the sample is in a sealed environment, and is not in contact with impurities. The obtained aluminum-doped and highly hydrated chromite single crystals are pure and have good chemical stability. They provide important experimental sample guarantees for the measurement of physical property parameters of aluminum-doped and highly hydrated chromite single crystals, especially for the study of crystal axis anisotropy and lattice preferred orientation of the physicochemical properties of chromite single crystals under high pressure.

[0055] Compared to previous synthetic methods for chromite single crystals, which employed methods such as high-temperature solid-phase sintering, polyvinyl acetate alcoholysis, carbonate co-precipitation, high-pressure powder hydrothermal method, metal alkoxide sol-gel method, microwave-activated solid-phase reaction method, and freeze-drying, the preparation method of the present invention offers significant advantages such as a simple operation process and short reaction time. The resulting chromite single crystals exhibit superior physical and chemical properties, including high purity, large size, and stable chemical properties. Most importantly, the chromite synthesis product has high aluminum content (8000-9000 ppm wt%) and high water content (250-450 ppm), and both aluminum and water contents are fully controllable. The large particle size of chromite single crystals can fully meet the sample requirements of single crystal mineral physical properties and spectroscopic experimental simulations under high temperature and high pressure conditions, such as electrical conductivity, synchrotron radiation X-ray diffraction, confocal Raman spectroscopy, and vacuum Fourier transform infrared spectroscopy on diamond pressure cell high-pressure equipment. This method provides important experimental sample guarantees for the measurement of physical property parameters of aluminum-doped and highly hydrated chromite single crystals, especially for the study of the preferred lattice orientation and crystal axis anisotropy of single crystal minerals under high pressure, breaking through the existing technical bottleneck of chromite single crystal synthesis. DETAILED DESCRIPTION

[0056] The specific preparation method of the present invention comprises:

[0057] The present invention uses solid transparent to translucent triangular ferrous carbonate crystals (purity: >99.99%), solid light grayish green to bluish basic chromium acetate crystalline powder (purity: >99.99%), solid aluminum distearate powder (purity: >99.99%), solid oxalic acid powder (purity: >99.99%), solid α-phase goethite powder (purity: >99%), solid aluminum hydroxide powder (purity: >99%), and liquid dilute nitric acid (concentration: 10%) as starting materials. The high-purity ferrous carbonate crystals selected as the starting material are a light grayish white solid with a glassy luster. They are chemically stable, are almost insoluble or slightly soluble in water, and readily dissolve in dilute acid solutions and produce foam. Triangular ferrous carbonate crystals are selected because of their excellent properties of stability and solubility in dilute acid, making them an excellent raw material for providing iron in synthetic chromite. The high-purity basic chromium acetate of the starting material selected by the present invention is a solid crystalline powder with a light gray green to bluish color. It is chemically stable, non-decomposable, non-toxic and soluble in water at room temperature. Basic chromium acetate can be used in industrial production such as high-purity metallic chromium, certain glazes, and colored glass. The basic chromium acetate crystalline powder is selected because it is easily decomposed in dilute acid solution and has the superior characteristics of strong chemical reactivity. Therefore, it is an excellent raw material for providing chromium element in artificially synthesized chromite. The high-purity aluminum distearate of the starting material selected by the present invention is a white powdery solid substance that is almost insoluble in solvents such as water, ethanol and ether, but is easily soluble in solvents such as turpentine, mineral oil, petroleum, kerosene, and benzene. When exposed to strong acid, it is easily decomposed into stearic acid and the corresponding aluminum salt. Aluminum distearate has a wide range of applications in industrial production and processing and manufacturing fields such as heat stabilizers, building waterproofing agents, emulsifiers, coatings, metal rust inhibitors, thickeners, and lubricants. Because it is soluble in dilute nitric acid solution, aluminum distearate is an excellent raw material for providing trace aluminum elements in artificially synthesized chromite. The high-purity solid oxalic acid of the starting material selected in the present invention is a chelating agent for metal substances. Its purpose is that oxalic acid powder has a great influence on the biological effectiveness of minerals and has a strong coordination effect. When oxalic acid combines with positive divalent iron ions, its solubility can be greatly reduced, thereby forming a complex sol of positive divalent iron ions in dilute nitric acid solution; at the same time, when oxalic acid combines with colored light metal cation aluminum, due to its coordination effect, a soluble colored light metal cation complex is formed. The solubility of the metal cation with positive trivalent aluminum in the acid solution will be significantly enhanced, so that it is fully dissolved in the dilute nitric acid solution. The high-purity solid α-phase goethite selected as the starting material of the present invention is a typical iron-containing hydrous mineral. One academic viewpoint of the predecessors is that: at a temperature of 270°C, α-phase goethite undergoes a dehydration reaction to directly generate hematite and release a large amount of water at the same time; another academic viewpoint is that: at a temperature of 238°C, α-phase goethite undergoes a first dehydration reaction to produce superstructured hematite [molecular formula: Fe (2-x / 3) (OH)x O (3-x) ], and the superstructured hematite undergoes a second dehydration reaction at a temperature of 800°C to generate hematite, while releasing a large amount of water. The high-purity solid aluminum hydroxide selected as the initial material of the present invention is a typical aluminum-rich white hydrated amorphous powder material, which undergoes three-step dehydration reactions at temperatures of 230°C, 300°C and 500°C, respectively, and the corresponding generated products are monohydrated aluminum oxide, crystalline aluminum oxide and corundum, respectively, while releasing a large amount of water. The dilute nitric acid (concentration: 10%) of the initial material selected by the present invention, if the nitric acid concentration is too low, due to its limited solubility, may cause ferrous carbonate crystals, basic chromium acetate crystalline powder, aluminum distearate powder and oxalic acid powder to remain; if the nitric acid concentration is too high, due to its enhanced oxidizing property, the ferrous carbonate crystals in the sample will directly undergo a rapid oxidation reaction or directly decompose, and produce thick smoke, which may bring certain risks to the preparation.

[0058] Step 1. Open the chemical fume hood, select a standard volume 100 ml volumetric flask, accurately weigh out 60 ml of 10% dilute nitric acid, place the glass pipette rod in a 500 ml notched beaker, and carefully transfer all the liquid dilute nitric acid into the beaker along the pipette rod. The notched beaker is chosen as the reaction container mainly because the beaker is not completely sealed after the glass watch glass is covered, and the gas generated can easily evaporate in the fume hood.

[0059] Step 2: Accurately weigh 5.0 g of high-purity transparent to translucent triangular ferrous carbonate crystals on a 10 μg high-precision analytical balance. Carefully add the crystals to a notched beaker containing 10% dilute nitric acid solution and place in a magnetic stirring rotor.

[0060] Step 3: Use a glass watch glass to cover the notched beaker containing the dilute nitric acid solution of solid ferrous carbonate crystals and place it on a high-temperature magnetic stirring hot plate in a fume hood. In order to fully dissolve the solid ferrous carbonate crystals of the initial material in the dilute nitric acid solution and simultaneously cause hydrolysis and acidification reactions, the reaction conditions are room temperature, 700 rpm, and reaction time for 72 hours.

[0061] Step 4: According to the stoichiometric ratio of chromite Fe(Cr,Al)2O4, 17.3584 g of high-purity solid basic chromium acetate crystalline powder and 650 mg of high-purity solid aluminum distearate powder were accurately weighed on a high-precision analytical balance, and carefully added to the dilute nitric acid solution containing ferrous carbonate.

[0062] Step 5: Place the dilute nitric acid solution containing solid ferrous carbonate crystals, solid basic chromium acetate crystalline powder, and solid aluminum distearate powder in a beaker and cover it with a glass watch glass to ensure that the gas generated by the reaction evaporates from the gap in the beaker and to prevent the dilute nitric acid solution of the initial material in the beaker from splashing out during the high-speed stirring process, thereby creating a danger and affecting the accuracy of the chromite single crystal synthesis.

[0063] Step 6, the initial dilute nitric acid mixed solution of sealing and the beaker of magnetic agitation rotor are housed, are placed on the high-temperature magnetic stirring hot plate in the fume hood, at normal temperature, 800 revs / min of rotating speeds and churning time 48 hours conditions, make the solid-state ferrous carbonate crystal of initial material, solid-state basic chromium acetate crystalline powder and solid-state aluminum distearate powder, all be dissolved in the mixed solution of dilute nitric acid solution, without any residual. Simultaneously, make NH3H2O, CH4, C2H2, CO2, CO and H2 and other volatile substances, more easily volatilize in the fume hood. Step 6: On a high-precision analytical balance, 2 grams of high-purity solid oxalic acid powder are accurately weighed, and the high-purity oxalic acid powder as an important metal chelating agent is added to a dilute nitric acid solution containing solid ferrous carbonate crystals, solid basic chromium acetate crystalline powder, and solid aluminum distearate powder. The purpose is that the oxalic acid powder has a great influence on the bioavailability of minerals and has a strong coordination effect. When oxalic acid combines with positive divalent iron ions, its solubility can be greatly reduced, thereby forming a complex sol of positive divalent iron ions in the dilute nitric acid solution. At the same time, when oxalic acid combines with colored light metal cation aluminum, due to its coordination effect, a soluble colored light metal cation complex is formed, and the solubility of the metal cation with positive trivalent aluminum in the acid solution will be significantly enhanced, so that it is fully dissolved in the dilute nitric acid solution.

[0064] Step 7. Place the notched beaker of the mixed solution back on the high-temperature magnetic stirring hot plate in the fume hood, cover it with a glass watch glass, and set the conditions of the high-temperature magnetic stirring hot plate to 80°C, 1000 rpm, and stirring time for 36 hours, so that all the initial reagents form a uniform sol under the action of the mixed solution of dilute nitric acid and oxalic acid.

[0065] Step 8: Remove the glass watch glass from the beaker and raise the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the notched beaker is completely evaporated.

[0066] Step 9: Remove the magnetic stirring rotor from the notched beaker on the high-temperature magnetic stirring hot plate and clean any powder sample adhered to its surface into the beaker. Carefully remove the mixed powder from the notched beaker with a spatula and place it in a graphite crucible. The purpose of using a graphite crucible is that the carbon that makes up the graphite crucible inevitably produces certain concentrations of carbon monoxide and carbon dioxide during the high-temperature calcination process, thereby controlling the oxygen fugacity of the chromite sample in the graphite crucible and ultimately constraining the valence state of the variable-valence metal cations, iron and chromium, in the chromite sample.

[0067] Step 10: The graphite crucible containing the powder mixture was heated to 1100°C in a muffle furnace at atmospheric pressure and high temperature at a relatively slow heating rate of 300°C / hour and held at that temperature for 5 hours. The relatively slow high-temperature calcination rate and longer hold time facilitated the control of the oxygen atmosphere within the graphite sample chamber and facilitated the complete removal of residual nitric acid, oxalic acid, and other organic matter from the powder mixture.

[0068] Step 11: Cool the mixed sample powder in the graphite crucible in the muffle furnace to room temperature at a cooling rate of 200°C / hour. Compared with the heating rate, a slower cooling rate is selected to more easily form a honeycomb-shaped loose sample powder. Carefully remove the mixed sample powder.

[0069] Step 12: Place the honeycomb-shaped loose chromite sample powder in an ultra-hard thickened corundum mortar and grind it thoroughly for 1 hour to obtain a fine-grained and homogenized powder experimental sample.

[0070] Step 13: Cold-press the uniform and fine-grained chromite powder sample mixture into three 10.0 mm x 3.0 mm sample discs using a stainless steel tablet press with a high-precision tungsten carbide grinding tool (10.0 mm x 10.0 mm). Stack the three cold-pressed sample mixtures vertically and carefully place them on the bottom of a graphite crucible.

[0071] Step 14. Use a high-speed electric drill to symmetrically drill two 1.0 mm diameter circular holes in the graphite crucible containing the three stacked samples. Carefully thread a 0.5 mm platinum-rhodium alloy wire through the two 1.0 mm symmetrical circular holes in the graphite crucible wall and suspend it in the center of a high-temperature oxygen atmosphere furnace. The ends of the platinum-rhodium wire connecting the graphite crucible are secured to a vertical, 0.6 mm diameter, four-hole alumina tube with an outer diameter of 5.0 mm and a length of 40 cm. The upper end of the four-hole alumina tube is secured in the center of a round lid that allows for easy insertion and removal of the furnace.

[0072] Step 15. Place a stainless steel container containing 3 liters of secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace in advance. The purpose is to pull the graphite crucible containing the sample directly out of the high-temperature oxygen atmosphere furnace at an extremely high temperature and quickly immerse it in the 3 liters of secondary deionized water in the cold stainless steel container to quickly cool it down. The main purpose is to avoid the valence elements iron and chromium from being oxidized / reduced again during the slow cooling of the furnace body, to achieve rapid quenching of the sample, and to completely preserve the glassy chromite sample.

[0073] Step 16: At the top of the high-temperature oxygen atmosphere furnace, a cylinder of argon inert gas and cylinders of carbon monoxide and carbon dioxide with adjustable ratios are connected. A barometer controls the amount of gas introduced into the sample chamber. During the high-temperature calcination of the sample, each gas can be switched and adjusted at any time via valves. The present invention uses argon inert gas to provide an absolutely reducing oxygen atmosphere when the furnace temperature is below 800°C.

[0074] The present invention uses carbon monoxide and carbon dioxide in an adjustable ratio. The purpose is to effectively control the oxygen fugacity of the sample during high-temperature calcination when the furnace temperature is higher than 800°C. If the furnace temperature is higher than 800°C, continuing to introduce argon inert gas will cause over-reduction in the sample chamber, which may cause the variable valence elements iron and chromium to be reduced to metallic iron and metallic chromium in sequence. Therefore, when the temperature is higher than 800°C, we use a mixed gas of carbon monoxide and carbon dioxide in an adjustable ratio to control the oxygen fugacity of the sample in the high-temperature oxygen atmosphere furnace chamber. The reaction principle is as follows: The method can well achieve the adjustment of any oxygen partial pressure in the sample chamber, thereby achieving the control of the valence state of the variable valence metal elements iron and chromium in the aluminum-doped and highly hydrated chromite single crystal.

[0075] The maximum rated temperature of the high-temperature oxygen atmosphere furnace body of the present invention is 1800° C. Turn on the circulating cooling water of the high-temperature oxygen atmosphere furnace to reduce the upper and lower temperatures of the furnace body to avoid excessive temperature of the entire furnace body, which may cause carbon monoxide and carbon dioxide leakage, thereby causing danger.

[0076] The present invention turns on a highly sensitive monitoring alarm for the concentrations of argon, carbon monoxide and carbon dioxide to avoid gas leakage during high-temperature calcination in an oxygen atmosphere furnace and ensure operator safety.

[0077] Step 17: Open the argon inert gas valve and rotate the pointer button controlled by the gas pressure gauge to continue inflating for 30 minutes to properly expel excess air from the sample chamber. Under the protection of argon inert gas, calcine the sample at a high temperature of 800°C at a heating rate of 400°C / hour.

[0078] Step 18. After the temperature inside the furnace reaches 800°C, quickly switch the carbon monoxide cylinder and the carbon dioxide gas control valve, and rotate the pointer button of the gas pressure gauge control to make the volume ratio of carbon monoxide and carbon dioxide passing through the sample oxygen atmosphere furnace reach 4:1. The purpose is that during the high-temperature calcination process, the carbon monoxide and carbon dioxide mixed gas with this volume ratio can well regulate the oxygen fugacity in the sample chamber.

[0079] Step 19: After the mixed gas flow of carbon monoxide and carbon dioxide, controlling the oxygen fugacity in the sample chamber at a volume ratio of 4:1, reaches a stable state (this step takes approximately 3–5 minutes), the sample chamber temperature within the furnace is then raised to 1430°C at a heating rate of 200°C / hour and calcined at this constant temperature for 15 minutes to melt the chromite into a glassy state. During the heating process in the high-temperature oxygen atmosphere furnace, two distinct heating rates of 400°C / hour and 200°C / hour are employed for the sample chamber within the temperature ranges of room temperature–800°C and 800°C–1430°C, respectively. This invention utilizes a slower heating rate as the sample chamber temperature rises. This facilitates the formation of stronger ionic bonds, such as those of Fe–O, Cr–O, and Al–O, in the aluminum-doped chromite; more precisely controls the temperature of the sample chamber within the high-temperature oxygen atmosphere furnace; and prevents overheating in localized areas of the furnace due to unbalanced heat transfer from the sample chamber, which can easily damage the oxygen atmosphere furnace's heating element.

[0080] The purpose of the high-temperature roasting process of the present invention in which a carbon monoxide and carbon dioxide mixed gas controls an oxygen atmosphere is to provide a purer chromite glassy material for synthesizing large-particle aluminum-doped and highly hydrated chromite single crystals. The high-temperature calcination under oxygen atmosphere conditions can better control the valence state of the variable-valence metal elements iron and chromium in the product. The relatively high calcination temperature of 1430°C can ensure that any small amount of volatile matter, nitric acid, oxalic acid, organic matter, and other substances that may affect sample preparation after high-temperature calcination in a muffle furnace are completely volatilized.

[0081] A relatively short calcination time of 15 minutes is used because chromite powder melts rapidly at temperatures above 1380°C. If the calcination time is too short, some residual starting powder may remain in the chromite melt, seriously affecting the chemical composition of the prepared chromite sample. This also hinders the sufficient chemical diffusion of metal cations such as iron, chromium, and aluminum, and the formation of strong ionic bonds such as Fe–O, Cr–O, and Al–O in chromite. If the calcination time is too short, the doped aluminum element may become unevenly distributed in the chromite, causing stratification and differentiation, which seriously affects the preparation results. If the calcination time is too short, the density of the product is reduced, making it difficult to form high-density chromite glass. However, calcination times longer than 15 minutes may result in excessive melting, causing the chromite sample to adhere firmly to the graphite crucible wall, making it difficult to clean and increasing sample preparation costs.

[0082] Step 20: After the sample is calcined at a constant temperature of 1430°C for 15 minutes, the graphite crucible containing the sample, the four-hole alumina tube, and the round cover on the furnace body are pulled out of the furnace body and directly immersed in a stainless steel container containing 3 liters of secondary deionized pure cold water to be rapidly quenched into chromite glass. The purpose of rapid quenching is to well preserve the glassy chromite sample with uniform composition at high temperature.

[0083] Step 21: Carefully remove the quenched glassy chromite sample from the graphite crucible and thoroughly grind it in a corundum mortar to form a fine-grained and uniform sample powder. Place the glassy chromite powder in a vacuum drying oven at 200°C for 12 hours.

[0084] Step 22: On a cold isostatic press, the chromite glass powder is cold pressed using a high-precision Φ4.0 mm (diameter) × 10.0 mm tungsten carbide grinding tool to form a cylindrical chromite sample with a size of Φ4.0 mm (diameter) × 4.0 mm (height).

[0085] In order to obtain chromite with high water content, the present invention adopts α-phase goethite powder (molecular formula: FeOOH) and aluminum hydroxide powder (molecular formula: Al(OH)3) in a weight ratio of 4:1 as the water source. The selection of a mixture of α-phase goethite and aluminum hydroxide as the water source is mainly based on the following considerations: First, α-phase goethite and aluminum hydroxide are both typical water-containing substances, and the dehydration temperature is relatively low. One academic view of the predecessors is that: α-phase goethite undergoes a dehydration reaction at a temperature of 270°C, directly generating hematite and releasing a large amount of water at the same time; another academic view is that: α-phase goethite undergoes the first dehydration reaction at a temperature of 238°C, and the product is superstructured hematite [molecular formula: Fe (2-x / 3) (OH) xO (3-x) ], while superstructured hematite undergoes a second dehydration reaction at 800°C, producing hematite and simultaneously releasing a large amount of water. Aluminum hydroxide, a typical aluminum-rich, white, amorphous powdery, hydrated solid, undergoes a three-step dehydration reaction at 230°C, 300°C, and 500°C, respectively, producing monohydrated aluminum oxide, crystalline aluminum oxide, and corundum, while simultaneously releasing a large amount of water. Therefore, this dehydration temperature condition can be achieved within the lower temperature range of the preparation process of aluminum-doped chromite single crystals under high temperature and high pressure conditions, fully ensuring that the aluminum-doped chromite single crystals are in a water environment for a sufficient period of time to ensure sufficient diffusion of sample lattice water and the formation of lattice occupancy. Secondly, α-phase goethite and aluminum hydroxide are both iron- and aluminum-rich substances, which can effectively control the iron and aluminum activities during the preparation of aluminum-doped and highly hydrated chromite single crystals in the sample cavity under high temperature and high pressure conditions. Finally, the dehydration products of the α-phase goethite and aluminum hydroxide water source material combination, placed at both ends of the sample in a 4:1 weight ratio, are hematite (Fe2O3) and corundum (Al2O3), which do not chemically react with the sample, ensuring the purity of the aluminum-doped and highly hydrous chromite single crystals. Furthermore, by adjusting the weight ratio of the α-phase goethite and aluminum hydroxide water source materials and the height of the corresponding water source sheet, the water content of the aluminum-doped and highly hydrous chromite single crystal samples can be adjusted.

[0086] Step 23: On a cold isostatic press, α-phase goethite powder and aluminum hydroxide powder are cold pressed in a weight ratio of 4:1 using a high-precision tungsten carbide grinding tool of Φ4.0mm (diameter) × 10.0mm to form two water source sheets of Φ4.0mm (diameter) × 0.1mm (height).

[0087] Step 24: Seal the cylindrical chromite sample (size: Φ4.0 mm (diameter) × 4.0 mm (height)) and two water source sheets (size: Φ4.0 mm (diameter) × 0.1 mm (height)) in sequence in a dual-chamber experimental sample chamber consisting of an inner sleeve - a graphite tube (size: Φ4.4 mm (outer diameter) × 4.4 mm (height), wall thickness 0.2 mm) and an outer sleeve - a gold-palladium alloy tube (size: Φ4.6 mm (outer diameter) × 4.6 mm (height), wall thickness 0.1 mm). In this invention, the aluminum-doped chromite sample is placed in the center of the inner graphite sleeve; the two water source sheets, consisting of α-phase goethite and aluminum hydroxide in a 4:1 weight ratio, are placed at symmetrical ends of the inner graphite sleeve, close to the sample.

[0088] In the present invention, the inner sleeve of the double-bag structure sample chamber adopts graphite as the sealing material, the main purpose of which is to control the oxygen fugacity value of carbon monoxide and carbon dioxide in the sample chamber to be maintained within the control range, and ultimately to achieve the goal of constraining the valence state of the variable valence metal elements iron and chromium in the chromite sample.

[0089] In the present invention, the outer sleeve of the double-capsule structure sample chamber adopts gold-palladium alloy as the sealing material, and the main purposes are: first, the gold-palladium alloy is used for sealing to isolate the material or element exchange between the sample and other surrounding pressure-transmitting materials, thereby effectively avoiding sample contamination during the preparation of chromite samples under high temperature and high pressure conditions; second, the gold-palladium alloy is used for sealing to effectively avoid water escaping from the sample tube during the preparation of chromite samples under high temperature and high pressure conditions; finally, the present invention adopts a double-capsule structure sample chamber composed of a graphite tube and a gold-palladium alloy tube to form a more closed oxygen atmosphere environment, better control the oxygen fugacity in the sample chamber, and thus more effectively constrain the valence state of the variable valence metal elements iron and chromium in the chromite sample.

[0090] Step 25. Chromite is one of the important iron-rich and chromium-rich oxide minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets. To realistically simulate the growth environment of chromite deep in the lower crust of the Earth and other terrestrial planets, and to invert the temperature and pressure conditions for the stable existence of the chromite mineral phase, a double-capsule structure sample chamber consisting of a graphite tube and a gold-palladium alloy tube was placed on a typical laboratory Kawai-1000t 6-8 type multi-faceted large cavity high-temperature and high-pressure equipment. The pressure and temperature were set to 0.5 GPa / hour and 10°C / minute, respectively. The pressure and temperature were raised to 3.0 GPa and 1100°C, respectively, and hot pressing sintering was carried out. The reaction time was kept at constant temperature and pressure for 72 hours.

[0091] The preparation process of the present invention with a high pressure of 3.0 GPa and a sintering temperature of 1100° C. is designed entirely based on the physical and chemical properties of chromite itself. The specific purposes are as follows: First, the preparation process of high temperature and high pressure conditions, relatively slow pressure rising and heating rates and long constant temperature and pressure reaction time can fully guarantee the complete mineral phase transformation from the initial chromite glass phase powder to the chromite crystal phase, and the final product chromite mineral phase can stably exist under the temperature and pressure conditions; secondly, the preparation process of high temperature and high pressure conditions, relatively slow pressure rising and heating rates and long constant temperature and pressure reaction time can significantly increase the self-diffusion and chemical diffusion coefficients of metal cations such as iron ions, chromium ions and aluminum ions, thereby realizing the isomorphic substitution of aluminum ions for metal chromium ions in chromite crystals, and the reaction is complete and no free aluminum element remains, thereby forming a perfect non-ferrous light metal element aluminum-doped chromite single crystal sample; secondly, the preparation process of high temperature and high pressure conditions, relatively slow pressure rising and heating rates and long constant temperature and pressure reaction time can fully ensure the formation of stable chemical bonds such as Fe-O, Cr-O, Al-O, thereby avoiding the doped aluminum element in The chromite is subjected to uneven distribution phenomena such as stratification and differentiation, thereby achieving uniform equiaxed aluminum-doped chromite single crystals. Secondly, the preparation process of high temperature and high pressure conditions, relatively slow pressure and heating rate, and long constant temperature and pressure reaction time causes a dehydration reaction of a water-containing combination of α-phase goethite and aluminum hydroxide in a weight ratio of 4:1 to produce a large amount of water. The final dehydration product is a mixed oxide of hematite and corundum. At the same time, the water is fully diffused in the aluminum-doped chromite single crystals in the sample chamber, thereby ensuring that the chromite sample has a sufficiently high water content. Finally, the preparation process of high temperature and high pressure conditions, relatively slow pressure and heating rate, and long constant temperature and pressure reaction time makes the aluminum element distribution in the final prepared product chromite more uniform, while increasing the density, strength and particle size of the product, thereby preparing aluminum-doped and high-water-content large-grain equiaxed chromite single crystal samples with excellent physical and chemical properties such as uniform element distribution, high mechanical strength and high density.

[0092] The temperature is precisely calibrated using two sets of high-temperature resistant tungsten-rhenium thermocouples. Tungsten-rhenium thermocouples have the advantages of good temperature-potential linearity, reliable thermal stability, and low price. They can achieve a temperature calibration range of 0-2300°C and are widely used in high-pressure mineral physics experiments, high-tech metallurgical industry, high-temperature electronic thermoelectric system structure engineering, space vehicles, nuclear reactors and other fields for ultra-high temperature calibration. Each set of tungsten-rhenium thermocouples is composed of two different tungsten-rhenium alloys, with a chemical composition of W. 95％ Re 5％ and W 74％ Re 26％. Put together one end of tungsten-rhenium thermocouple wires of different materials with a diameter of 0.1 mm, and use a vise to hang them into a twisted shape; connect the other end of the tungsten-rhenium thermocouple wires of different materials with a diameter of 0.1 mm to the positive and negative poles of a high-power welding regulated DC power supply. Adjust the output current control knob of the high-power welding regulated DC power supply to pass a large current through the wire, so that the twisted tungsten-rhenium high-temperature thermocouple wire is completely immersed in the saturated sodium chloride solution, melted, and welded into a ball, and the oxide layer on the surface of the spherical thermocouple wire is removed. Using the same technical solution as above, two groups of hot tungsten-rhenium thermocouples were prepared, and each group of tungsten-rhenium thermocouples was symmetrically placed at the upper and lower ends of the double-capsule structure sample chamber composed of a graphite tube and a gold-palladium alloy tube. The present invention adopts a technique of placing tungsten-rhenium dual thermocouples at the upper and lower ends of the sample chamber. This technology can realize accurate calibration of the temperature in the sample chamber and accurately indicate the temperature gradient at the upper and lower ends of the sample chamber, ensuring that the sample is in a stable constant temperature zone during the synthesis process of the chromite sample.

[0093] Step 26: After maintaining constant temperature and pressure at 3.0 GPa and 1100°C for 72 hours, the temperature in the sample chamber is reduced from 1100°C to 800°C at a cooling rate of 3°C / min and maintained at this temperature for 1 hour; the temperature in the sample chamber is then reduced from 800°C to room temperature at a cooling rate of 5°C / min. The use of a step-wise cooling method and a slower constant pressure cooling rate relative to the sample preparation heating rate (10°C / min) further enhances the superior physical and chemical properties of the aluminum-doped chromite single crystal sample, which exhibits uniform aluminum distribution, high mechanical strength, and high density. This method completely avoids the generation of uneven stress in the sample due to an excessively rapid cooling rate, which in turn leads to cracks and breakage in the chromite crystal. Furthermore, this preparation process is more conducive to the growth of large-grained chromite single crystals, thereby enabling the preparation of large-grained chromite single crystal samples of 100 micrometers.

[0094] After the temperature in the sample chamber has dropped to room temperature, step 27 reduces the pressure in the sample chamber from 3.0 GPa to atmospheric pressure at a rate of 0.5 GPa / hour. Furthermore, the present invention utilizes a hot-pressing sintering process to prepare aluminum-doped, highly hydrated chromite single crystals, ensuring a pure preparation process without any potential impurities introduced from the sample itself or during high-pressure sample assembly.

[0095] Step 28: After the high-temperature, high-pressure preparation reaction is complete, the sample is removed from the Kawai-1000t, a typical 6-8-inch multifaceted, large-cavity, high-temperature, high-pressure apparatus. The graphite tube and gold-palladium alloy tube enclosing the sample's double-bladder structure are carefully removed. Using a high-precision diamond wire cutter, the cylindrical sample is cut down the middle. The chromite single crystal is then selected under a high-precision Olympus microscope at 20x magnification.

[0096] The obtained chromite single crystal is a single phase without any other impurity phase; the electron probe (EPMA) detection result shows that the molecular formula of the obtained chromite single crystal is FeCr2O4; the multi-functional ion mass spectrometer (ICP-MS) detection result shows that the aluminum content in the obtained chromite single crystal is 8583 ppm wt%; the vacuum Fourier transform infrared spectroscopy (FT-IR) detection result shows that the water content of the obtained chromite single crystal sample is 401 ppm wt, and the water content is relatively high.

[0097] The obtained aluminum-doped and high-water-content chromite single crystal is of a cubic crystal system, the space group is Fd3m (no. 227), and the lattice parameter is α = β = γ = 90°, and the unit cell volume is The average particle size is 172 microns, and the maximum particle size is 545 microns.

[0098] The aluminum-doped and high-water-content chromite single crystal obtained by the application has high purity, large particle size, stable chemical properties, high mechanical strength and other superior performances, and more importantly, the aluminum content is high (8583 ppm wt%), and the aluminum content in the chromite single crystal can be completely controlled. By changing the chemical reagent amount of the added initial solid-state high-purity aluminum bis-stearate powder from 605.8665 mg to 681.5998 mg, the corresponding aluminum content in the finally obtained aluminum-doped and high-water-content chromite single crystal sample can be realized from 8000 ppm wt% to 9000 ppm wt%; by changing the weight ratio of the water-containing substance alpha-phase goethite powder and the aluminum hydroxide powder providing the water source and adjusting the different heights of the corresponding two water source sheets, the total water amount generated by the dehydration reaction of the water-containing substance enclosed in the double-capsule structure sample bin composed of the graphite tube and the gold-palladium alloy tube can be controlled, and finally the water content in the chromite single crystal can be adjusted. The obtained aluminum-doped and high-water-content chromite single crystal sample can completely meet the needs of physical experiment simulation of minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets under high temperature and high pressure conditions, and breaks through the technical bottleneck of the existing chromite single crystal synthesis, and provides important experimental sample support for the study of the lattice preferred orientation and the crystal axis anisotropy of the single crystal minerals in the lower crust and upper mantle regions of the Earth and other terrestrial planets under high temperature and high pressure conditions.

Claims

1. A method for preparing aluminum-doped and highly hydrous chromite single crystals under high temperature and high pressure, characterized by: The method comprises: preparing a cylindrical chromite sample using solid triangular ferrous carbonate crystals, solid basic chromium acetate crystal powder, solid aluminum distearate powder, solid oxalic acid powder, solid α-phase goethite powder, solid aluminum hydroxide powder and liquid dilute nitric acid as starting materials; preparing two water source sheets by combining the α-phase goethite powder and the aluminum hydroxide powder in a weight ratio of 4:1; placing the two water source sheets at both ends of the cylindrical chromite sample and placing the sample in a double-capsule structure sample chamber for high-temperature and high-pressure reaction to obtain a chromite single crystal; the method for preparing the cylindrical chromite sample comprises: Step 1: Weigh out 60 ml of 10% dilute nitric acid and transfer it to a notched beaker; Step 2: Weigh out 5.0 g of triangular ferrous carbonate crystals and add them to a notched beaker, then place a magnetic stirring rotor in it; Step 3: Cover the notched beaker with a glass watch glass and react at room temperature at 700 rpm for 72 hours. Step 4: According to the stoichiometric ratio of chromite Fe(Cr,Al)2O4, 17.3584 g of solid basic chromium acetate crystalline powder and 650 mg of solid aluminum distearate powder were weighed and added to a notched beaker respectively; Step 4: Cover the notched beaker with a glass watch glass; Step 5: Place the notched beaker on a high-temperature magnetic stirring plate in a fume hood and stir at 800 rpm for 48 hours at room temperature. Step 6: Weigh out 2 grams of solid oxalic acid powder and add it to the notched beaker; Step 7: Place the notched beaker on a high-temperature magnetic stirring plate in a fume hood, cover with a glass watch glass, set the high-temperature magnetic stirring plate to 80 °C, and stir at 1000 rpm for 36 hours. Step 8: Remove the glass watch glass from the beaker and increase the temperature of the high-temperature magnetic stirring hot plate to 110°C until the mixed solution in the notched beaker is completely evaporated; Step 9: Take out all the mixed powders in the notched beaker and place them in a graphite crucible; Step 10: The graphite crucible containing the mixture powder was placed in a muffle furnace at normal pressure and high temperature, and the temperature was increased to 1100 ° C at a heating rate of 300 ° C / hour and kept at this temperature for 5 hours; Step 11, cooling the mixed sample powder to room temperature at a cooling rate of 200 °C / hour; Step 12: Grind the mixed sample powder in a corundum mortar for 1 hour; Step 13: cold-press the ground mixed sample powder into three sample discs of Φ 10.0 mm × 3.0 mm, and stack the three cold-pressed sample mixtures vertically together and place them at the bottom of a graphite crucible; Step 14: Hang the graphite crucible in the middle of the high-temperature oxygen atmosphere furnace; Step 15: Place a stainless steel container filled with secondary deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace; Step 16: Connect the top of the high-temperature oxygen atmosphere furnace to an argon inert gas cylinder and carbon monoxide and carbon dioxide cylinders with adjustable ratios; Step 17: Open the argon inert gas valve and continue to inflate for 30 minutes. Under the protection of argon inert gas, calcinate the sample to 800°C at a heating rate of 400°C / hour. Step 18: After the furnace temperature reaches 800°C, switch the carbon monoxide cylinder and carbon dioxide gas control valves so that the volume ratio of carbon monoxide to carbon dioxide in the sample oxygen atmosphere furnace reaches 4:1; Step 19: After the mixed gas flow of carbon monoxide and carbon dioxide in a volume ratio of 4:1 to control the oxygen fugacity in the sample chamber reaches a stable state, the temperature of the sample chamber in the furnace is increased to 1430°C at a heating rate of 200°C / hour and calcined at a constant temperature for 15 minutes; Step 20: After constant temperature roasting at 1430°C for 15 minutes, the graphite crucible containing the sample is pulled out of the furnace body and directly immersed in a stainless steel container to quench into chromite glass; Step 21: remove the quenched chromite glass from the graphite crucible, grind it in a corundum mortar, and place the glassy chromite powder in a vacuum drying oven at 200° C. for 12 hours; Step 22: cold-press the chromite glass powder on a cold isostatic press to form a cylindrical chromite sample with a diameter of Φ 4.0 mm and a height of 4.0 mm.

2. The method for preparing aluminum-doped and high-hydrous chromite single crystals under high temperature and high pressure according to claim 1, characterized in that: The purity of solid triangular ferrous carbonate crystals is >99.99%, the purity of solid basic chromium acetate crystal powder is >99.99%, the purity of solid aluminum distearate powder is >99.99%, the purity of solid oxalic acid powder is >99.99%, the purity of solid α-phase goethite powder is >99%, the purity of solid aluminum hydroxide powder is >99% and the concentration of liquid dilute nitric acid is 10%.

3. The method for preparing aluminum-doped and high-hydrous chromite single crystals under high temperature and high pressure according to claim 1, characterized in that: The method for preparing two water source sheets by mixing α-phase goethite powder and aluminum hydroxide powder in a weight ratio of 4:1 is as follows: α-phase goethite powder and aluminum hydroxide powder are cold-pressed into two water source sheets with a diameter of Φ 4.0 mm and a height of 0.1 mm using a tungsten carbide grinding tool with a diameter of Φ 4.0 mm and a height of 10.0 mm according to a weight ratio of 4:

1.

4. The method for preparing aluminum-doped and high-hydrous chromite single crystals under high temperature and high pressure according to claim 1, characterized in that: The method of placing two water source sheets at both ends of a cylindrical chromite sample and placing the sample in a double-capsule structure sample chamber for high-temperature and high-pressure reaction to obtain a chromite single crystal includes: Step 24: Seal the cylindrical chromite sample and two water source tablets in sequence in a double-sleeved experimental sample chamber with a graphite inner casing and a gold-palladium alloy outer casing; place the chromite cylindrical sample in the center of the inner graphite casing; and place the water source tablets at both ends of the inner graphite casing close to the sample. Step 25. Place the double-capsule structure sample chamber on a typical Kawai‒1000t 6-8 type multi-faceted large-cavity high-temperature and high-pressure equipment in the laboratory. Set the pressure and temperature increase rates to 0.5 GPa / hour and 10°C / minute, respectively. Raise the pressure and temperature to 3.0 GPa and 1100°C, respectively, and perform hot pressing sintering. The reaction time is 72 hours at constant temperature and pressure. Step 26: After maintaining constant temperature and pressure at 3.0 GPa and 1100°C for 72 hours, the temperature in the sample chamber was reduced from 1100°C to 800°C at a cooling rate of 3°C / min and maintained at that temperature for 1 hour; then, the temperature in the sample chamber was reduced from 800°C to room temperature at a cooling rate of 5°C / min. Step 27: After the temperature in the sample chamber drops to room temperature, the pressure in the sample chamber is reduced from 3.0 GPa to atmospheric pressure at a pressure reduction rate of 0.5 GPa / hour; Step 28. After the high-temperature and high-pressure preparation reaction is completed, the sample is removed from the Kawai‒1000t typical 6-8 type multi-faceted large-cavity high-temperature and high-pressure equipment. The graphite tube and gold-palladium alloy tube of the double-capsule structure sample chamber that encloses the sample are removed. The cylindrical sample is cut in the middle using a diamond wire cutting instrument, and the chromite single crystal is selected under an Olympus microscope.

5. The method for preparing aluminum-doped and high-hydrous chromite single crystals under high temperature and high pressure according to claim 1, characterized in that: During high temperature and high pressure reactions, the temperature is calibrated using two sets of tungsten-rhenium thermocouples. Each set of tungsten-rhenium thermocouples is made of two different tungsten-rhenium alloys, with a chemical composition of W. 95% Re 5% and W 74% Re 26% Each set of tungsten-rhenium thermocouples is symmetrically placed at the upper and lower ends of a double-capsule structure sample chamber composed of a graphite tube and a gold-palladium alloy tube.