Preparation method of calcium-doped high-hydrated zinc-iron spinel single crystal under high temperature and high pressure

Abstract

The application discloses a preparation method of calcium-doped high-hydrated zinc-iron spinel monocrystal under high temperature and high pressure, and the method comprises the following steps: preparing a cylindrical zinc-iron spinel sample by taking solid-state basic zinc carbonate inorganic compound powder, solid-state transparent red-brown flaky iron (III) citrate crystals, solid-state oxalic acid powder, solid-state calcium stearate powder, solid-state zinc hydroxide powder, solid-state slaked lime powder and liquid dilute nitric acid as starting raw materials; preparing water source pieces by taking zinc hydroxide powder and slaked lime powder with a weight ratio of 4:1 as water sources; and performing high-temperature and high-pressure reaction on the cylindrical zinc-iron spinel sample and two water source pieces which are sequentially sealed in an inner sleeve-graphite tube to obtain high-hydrated zinc-iron spinel monocrystal. The method completely solves the blank of the preparation technology of calcium-doped high-hydrated zinc-iron spinel large-granular monocrystal under the current high-temperature and high-pressure conditions, so as to obtain experimental samples of calcium-doped high-hydrated zinc-iron spinel large-granular monocrystal.

Description

Technical Field

[0001] This invention belongs to the technical field of mineral single crystal sample synthesis, and particularly relates to a method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure. Background Technology

[0002] Zinc-iron spinel, an important end-member component of the magnetite subgroup of spinel minerals, has the chemical formula ZnFe₂O₄ and is an important oxide mineral rich in both zinc and iron. The percentage of the mineralogy of zinc-iron spinel oxides can be expressed as: ZnO / (ZnO+Fe₂O₃) = 33.8% and Fe₂O₃ / (ZnO+Fe₂O₃) = 66.2%. Typically, zinc-iron spinel is the most typical oxide mineral with a normal spinel structure in nature. In the corresponding unit cell, the cubic close-packed proportion of zinc-iron spinel with an anti-spinel structure is 0, exhibiting extremely pronounced physicochemical properties of a normal spinel structure.

[0003] Existing geological data indicates that exposed zinc-iron spinel single crystals are typically bluish-gray in color, generally exhibiting optical mineralogical properties such as reddish-brown to black streaks, conchoidal fracture, lack of cleavage, slight magnetism, and metallic luster. Zinc-iron spinel single crystals also belong to the typical isometric crystal system common to spinel group minerals, occurring as octahedral or granular aggregates with rounded edges, and a relative density of 5.07–5.22 g / cm³. 3 In nature, zinc-iron spinel single crystals are mainly found in outstanding metasomatisms of intrusive rocks—dolomite, intrusive rocks—mafic limestone, etc., and are often associated or coexisting with deep mantle silicate minerals such as mafic olivine and diopside. Zinc-iron spinel minerals are also found in silica-poor and aluminum-rich argillaceous rocks in typical regional geological tectonic units—hydrothermal contact metamorphic zones. As accessory minerals, zinc-iron spinel minerals are often hosted in rock bodies with geological tectonic environments such as basic rocks, supermagmatic rocks, or sandstones. For example, large areas of zinc-iron spinel single crystals are exposed in the basic extrusive magmatic rocks of eastern Anhui Province and the ultrabasic rocks of the Qinling-Dabie Mountains region in eastern China. Zinc-iron spinel single crystals from these regional geological bodies are brightly colored and have relatively large crystal sizes, and are often used as gemstones. Furthermore, due to its high melting point, zinc-iron spinel also has wide applications in the preparation and processing of refractory materials.

[0004] In the crystal structure of zinc-iron spinel, calcium, an alkaline earth metal located in the fourth period and group IIA, readily occupies tetrahedral positions, leading to isomorphic substitution of divalent cations at the A-site. Since the zinc and doped calcium elements at the lattice positions in zinc-iron spinel have the same +2 valence, this isomorphic substitution is classified as equivalent isomorphic substitution. Calcium constitutes 4.15% of the Earth's crust, ranking fifth among the ten most abundant elements. Important calcium-bearing minerals are widely exposed on the Earth's surface, and are numerous and widely distributed. For example, there are many calcium-bearing minerals, such as limestone (CaCO3), a trigonal carbonate mineral; gypsum (CaSO4·2H2O), a monoclinic sulfate mineral; dolomite (CaMg(CO3)2), a trigonal carbonate mineral; fluorite (CaF2), a cubic halide mineral; and apatite (Ca5(PO4)3F), a hexagonal oxygen-bearing salt mineral. These minerals are found in various mineral groups with diverse crystal structures. Furthermore, calcium is abundant in pearls, corals, seawater, animal shells, and soil. Recently, using a 1200-ton Kawai–1000 multi-faceted large-cavity press and a diamond-cavity high-temperature and high-pressure experimental setup, based on experimental results from single-crystal synchrotron radiation X-ray diffraction and high-resolution scanning electron microscopy under high pressure, it was discovered that under 61 GPa high pressure, calcium-rich spinel group minerals—calcium iron aluminum spinel (chemical formula: CaFe)—can exhibit high calcium content and high viscosity. 1.2 Al 0.8 O4 can exist stably in the deep Earth – middle lower mantle environment, and is the most important newly discovered carrier of aluminum-rich mineral phases in the Earth's internal spheres.

[0005] Zinc-iron spinel, with its spinel structure, does not contain water molecules or hydroxyl groups in its molecular structure, exhibiting the characteristics of a nominally anhydrous mineral. However, previous experimental results on the water solubility of spinel under high temperature and high pressure infrared spectra show that the amount of water that spinel can dissolve can reach hundreds of ppm. Water is one of the most important volatile components in the major spheres of the Earth's interior, especially in the mantle transition zone from 410 km to 660 km (corresponding to pressures and temperatures of 16.0-23.0 GPa and 1450-1800℃). Existing experimental studies on the physical and spectroscopic properties of minerals and rocks under high temperature and high pressure conditions, including electrical conductivity, Brillouin scattering elastic wave velocity, thermal diffusivity, thermal conductivity, and vacuum Fourier transform infrared spectroscopy, indicate that trace amounts of water in nominally anhydrous minerals can increase the physical and spectroscopic properties of minerals and rocks by several orders of magnitude, having a significant impact on their physical properties. The main methods used in the artificial synthesis of zinc-iron spinel in domestic and international laboratory materials science fields include: acid hydrolysis of polyvinyl alcohol, ammonia chemical precipitation, high-pressure hydrothermal synthesis, high-temperature solid-state sintering, inorganic salt sol-gel method, and freeze-drying. These existing synthesis techniques mostly involve simple solution chemical reactions or direct particle grinding of sample powders followed by high-temperature sintering, which are suitable for preparing nanoscale zinc-iron spinel crystals. However, in high-temperature and high-pressure experimental geoscience research, micron-sized or larger mineral single-crystal samples are often required. Clearly, the nanoscale zinc-iron spinel samples obtained through previous material synthesis methods fail to meet the minimum particle size requirements, and no effective synthesis method has yet been found. Previously, many geoscience researchers have also used natural zinc-iron spinel samples instead of artificially synthesized samples to meet the needs of high-temperature and high-pressure experimental geoscience research. However, these natural samples suffer from significant drawbacks due to the uneven distribution of trace element calcium. Therefore, it is particularly urgent to effectively synthesize a large-particle calcium-doped, high-hydration zinc-iron spinel single crystal that meets the needs of geoscience research in various high-temperature and high-pressure laboratory simulations, especially for the study of the optimal orientation of the zinc-iron spinel single crystal mineral lattice and the anisotropy of the crystal axis 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 calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure, so as to completely solve the current technical gap in the preparation of large-particle calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure conditions, and to obtain experimental samples of large-particle calcium-doped high-hydration zinc-iron spinel single crystals.

[0007] The technical solution of this invention is:

[0008] A method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure involves using solid basic zinc carbonate inorganic compound powder, solid transparent reddish-brown flake-shaped iron (III) citrate crystals, solid oxalic acid powder, solid calcium stearate powder, solid zinc hydroxide powder, solid quicklime powder, and liquid dilute nitric acid as starting materials to prepare cylindrical zinc-iron spinel samples; using zinc hydroxide powder and quicklime powder in a weight ratio of 4:1 as water sources to prepare water source sheets; and sealing the cylindrical zinc-iron spinel samples and the two water source sheets sequentially in an inner sleeve-graphite tube before conducting a high-temperature and high-pressure reaction to obtain high-hydration zinc-iron spinel single crystals.

[0009] The following substances have the following purity levels: basic zinc carbonate inorganic compound powder (>99.99%), solid transparent reddish-brown flaky ferric citrate (III) crystals (>99.99%), solid oxalic acid powder (>99.99%), solid fine white calcium stearate powder (>99.99%), solid zinc hydroxide powder (>99%), solid slaked lime powder (>99%), and liquid dilute nitric acid (10%).

[0010] The preparation method of the cylindrical zinc-iron spinel sample includes:

[0011] Step 1: Weigh out 60 ml of 10% dilute nitric acid and place it in a 500 ml notched beaker.

[0012] Step 2: Weigh out 5.0 grams of basic zinc carbonate inorganic compound powder and add it to a notched beaker, then place a magnetic stirring rotor inside.

[0013] Step 3: Cover the notched beaker with a glass watch glass and react at 700 rpm for 72 hours at room temperature.

[0014] Step 4: According to the stoichiometric ratio of zinc-iron spinel (Zn,Ca)Fe2O4, weigh out 22.3074 g of solid iron citrate (III) crystals and 120 mg of solid calcium stearate powder and add them to a dilute nitric acid solution containing basic zinc carbonate.

[0015] Step 5: Cover the notched beaker with a glass watch glass.

[0016] Step 6: Place the notched beaker on a high-temperature magnetic stirring plate and stir at 800 rpm for 48 hours at room temperature.

[0017] Step 7: Weigh out 2 grams of solid oxalic acid powder and add it to the notched beaker;

[0018] Step 8: Place the notched beaker on the high-temperature magnetic stirring plate in the fume hood, cover it with a glass watch glass, and stir at 80°C and 1000 rpm for 36 hours.

[0019] Step 9: Remove the glass watch glass from the beaker, and adjust the temperature of the high-temperature magnetic stirring plate to 110°C until the mixed solution in the entire notched beaker is completely evaporated.

[0020] Step 10: Remove the magnetic stirring rotor from the notched beaker on the high-temperature magnetic stirring hot plate, remove all the mixed powder from the notched beaker, and place it in the graphite crucible;

[0021] Step 11: Place the graphite crucible containing the mixed powder into a muffle furnace under normal pressure and high temperature conditions, raise the temperature to 1100°C at a heating rate of 300°C / hour, and hold the temperature for 5 hours.

[0022] Step 12: Cool to room temperature at a rate of 200℃ / hour, and remove the mixture sample powder;

[0023] Step 13: Place the mixed sample powder in a corundum mortar and grind for 1 hour;

[0024] Step 14: Cold press the ground mixture sample powder into 3 sample discs, and place the 3 cold-pressed sample mixture discs vertically together at the bottom of the graphite crucible;

[0025] Step 15: Suspend the graphite crucible containing the three stacked samples in the center of the high-temperature oxygen atmosphere furnace.

[0026] Step 16: Place a stainless steel container filled with deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace.

[0027] Step 17: Connect the top of the high-temperature oxygen atmosphere furnace to an argon inert gas cylinder and a carbon monoxide and carbon dioxide cylinder with adjustable ratios.

[0028] Step 18: Open the argon inert gas valve and continue to purge for 30 minutes. Under the protection of argon inert gas, calcine the sample to 800°C at a heating rate of 400°C / hour.

[0029] Step 19: After the temperature inside the furnace reaches 800℃, switch the carbon monoxide gas cylinder and carbon dioxide gas control valve to make the volume ratio of carbon monoxide and carbon dioxide passing through the sample oxygen atmosphere furnace reach 4:1.

[0030] Step 20: Increase the temperature of the sample chamber inside the furnace to 1420℃ at a heating rate of 200℃ / hour, and bake at a constant temperature for 15 minutes.

[0031] Step 21: After the sample is calcined at a constant temperature of 1420℃ for 15 minutes, the graphite crucible containing the sample is pulled out of the furnace and directly immersed in a stainless steel container to quench into zinc-iron spinel glass.

[0032] Step 22: Take the quenched glassy zinc-iron spinel sample out of the graphite crucible, grind it in a corundum mortar, and then place the glassy zinc-iron spinel powder in a vacuum drying oven at 200°C for 12 hours.

[0033] Step 23: On a cold isostatic press, zinc-iron spinel glass powder is cold-pressed into cylindrical zinc-iron spinel samples with diameter Φ4.0mm × height 4.0mm using a tungsten carbide mold.

[0034] The method for preparing water source sheets using zinc hydroxide powder and quicklime powder in a weight ratio of 4:1 is as follows: On a cold isostatic press, zinc hydroxide powder and quicklime powder are cold-pressed in a weight ratio of 4:1 using a tungsten carbide mold to form two water source sheets with a diameter of Φ4.0mm × 0.1mm (height).

[0035] Methods for obtaining high-water-content zinc-iron spinel single crystals by sequentially sealing cylindrical zinc-iron spinel samples and two water source plates within an inner sleeve-graphite tube and then subjecting them to a high-temperature, high-pressure reaction include:

[0036] Step 25: Seal the cylindrical zinc-iron spinel sample and two water source plates in sequence in a double-capsule experimental sample chamber with an inner casing of graphite tube and an outer casing of gold-palladium alloy tube.

[0037] Step 26: Place the double-capsule sample chamber on a typical 6–8 type multi-faceted top large cavity high temperature and high pressure equipment of Kawai-1000t in the laboratory. Set the pressure increase rate and the temperature increase rate to 0.5 GPa / hour and 10℃ / minute, respectively. Under the conditions of raising the pressure and temperature to 3.0 GPa and 1100℃, respectively, hot pressing sintering is carried out, and the reaction time is constant temperature and pressure for 72 hours.

[0038] Step 27: Reduce the temperature inside the sample chamber from 1100℃ to 800℃ at a cooling rate of 3℃ / min, and hold the temperature for 1 hour; then reduce the temperature inside the sample chamber from 800℃ to room temperature at a cooling rate of 5℃ / min.

[0039] Step 28: After the temperature inside the sample chamber drops to room temperature, reduce the pressure inside the sample chamber from 3.0 GPa to atmospheric pressure at a depressurization rate of 0.5 GPa / hour.

[0040] Step 29: Remove the sample from the typical 6–8 type multi-faceted top large cavity high temperature and high pressure equipment of Kawai-1000t, remove the graphite tube and gold-palladium alloy tube of the double-capsule sample chamber that encloses the sample, cut the cylindrical sample from the middle with a diamond wire cutter, and select the zinc-iron spinel single crystal under an Olympus microscope.

[0041] During the high-temperature and high-pressure reaction, two sets of tungsten-rhenium thermocouples were used for temperature calibration; each set of tungsten-rhenium thermocouples was composed of two different tungsten-rhenium alloys with the chemical composition 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 the double-capsule sample chamber composed of graphite tubes and gold-palladium alloy tubes.

[0042] The beneficial effects of this invention are:

[0043] This invention organically combines backgrounds from related Earth science disciplines such as crystallography, mineralogy, mineral crystal structure, igneous petrology, elemental geochemistry, structural geology, mineral X-ray diffraction, high-pressure mineral physics, mineral materials science, mining geology, nanomaterials science, rock physics, rock-forming mineralogy, advanced geochemistry, and mineral materials processing. It utilizes a typical 6–8 type multi-faceted top large-cavity high-temperature and high-pressure laboratory Kawai-1000t equipment to simulate the formation process of calcium-doped, high-hydration zinc-iron spinel single crystals under high-temperature and high-pressure conditions. The main chemical reaction equations involved in this invention are:

[0044] [3Zn(OH)2·2ZnCO3]+10HNO3→5Zn(NO3)2+2CO2+8H2O

[0045] Zn(NO3)2+2C6H5FeO7→ZnFe2O4+2(NH3·H2O)+10CO+2CO2

[0046] ZnFe2O4+C 36 H 70 CaO4→(Zn,Ca)Fe2O4+10CH4+12C2H2+2CO2+3H2

[0047] Zn(OH)₂→ZnO+H₂O

[0048] Ca(OH)₂→CaO+H₂O

[0049] In this invention, the initial raw material selected, basic zinc carbonate [chemical formula: 3Zn(OH)2·2ZnCO3], is a white, fine, amorphous inorganic compound powder. It is odorless and tasteless, readily soluble in dilute acids and sodium hydroxide, slightly soluble in ammonia, and insoluble in water and alcohol. It is mainly used in industrial production in fields such as latex industry, rayon production, skin protectants, pharmaceuticals, and feed additives. Basic zinc carbonate solid powder is chosen because of its stable properties and superior solubility in dilute acids, making it an excellent raw material for providing zinc in artificially synthesized zinc-iron spinel. The initial raw material, ferric citrate (III) [also known as: triferric citrate, chemical formula: C6H5FeO7], is a transparent, reddish-brown, thin, flaky crystalline solid inorganic compound, insoluble in ethanol, but soluble in water and dilute acids. Ferric citrate (III), a typical edible citrate crystal, is used in food science research as an iron fortifier, acidulant, and nutritional supplement; and in medical research as a radioactive pharmaceutical agent for monitoring iron metabolism abnormalities and hematopoietic function. Ferric citrate (III) crystals are chosen because of their easy decomposition in dilute acid solutions and strong chemical reactivity, making them an excellent raw material for providing iron in the artificial synthesis of zinc-iron spinel. Initial raw material: calcium stearate [chemical formula: C...] 36 H 70 CaO4, also known as calcium stearate, is a fine, white, fluffy, and slippery powdery solid of an organic compound. It is almost insoluble in solvents such as water, acetone, and chloroform, and is hygroscopic in air. Calcium stearate decomposes in acidic solutions and is widely used in the industrial production and preparation of plastics, paints, coatings, lubricants, and mold release agents. This invention selects fine, white calcium stearate powder, which is soluble in dilute nitric acid solution, making it an excellent raw material for providing trace element calcium in artificially synthesized zinc-iron spinel. The selected initial raw material, zinc hydroxide [molecular formula: Zn(OH)2], is a typical zinc-rich powdery hydrated solid that undergoes a dehydration reaction at 125°C to produce zinc oxide (ZnO), releasing a large amount of water in the process. The selected initial raw material, slaked lime [molecular formula: Ca(OH)2], is a typical calcium-rich, powdery, hydrated solid. At 580℃, slaked lime undergoes a dehydration reaction to produce quicklime (CaO), releasing a large amount of water in the process. In a high-pressure sample chamber, zinc hydroxide and slaked lime, in a specific ratio, undergo a dehydration reaction under high temperature and pressure, producing a large amount of water. This provides an excellent water source for the synthesis of calcium-doped, high-hydration zinc-iron spinel single crystals. The chemical reaction products involved in this invention, including NH3·H2O, CH4, C2H2, CO2, CO, and H2, are all volatile substances obtained at high temperatures.

[0050] This invention aims to synthesize large-particle single crystals of calcium-doped zinc-iron spinel with high water content. The synthesized samples contain calcium-doped zinc-iron spinel single crystals suitable for the development and comprehensive utilization of zinc mineral resources. These crystals will be widely applied in diagenetic and mineralization experimental simulation studies of the physicochemical properties of minerals and rocks under high temperature and high pressure conditions. Compared to naturally exposed zinc-iron spinel samples, which may contain impurities such as cobalt, calcium, and magnesium ions, the preparation process of calcium-doped and high-water-content zinc-iron spinel single crystals in this invention utilizes a pure laboratory environment. The samples are kept in a sealed environment, free from contact with impurities. The resulting calcium-doped and high-water-content zinc-iron spinel single crystals are pure substances with good chemical stability. This provides crucial experimental sample support for measuring the physical properties of calcium-doped and high-water-content zinc-iron spinel single crystals, especially for investigating the anisotropy of crystal axes and optimal lattice orientation of zinc-iron spinel single crystals under high pressure.

[0051] Compared to previous methods for synthesizing zinc-iron spinel single crystals, such as acid hydrolysis with polyvinyl alcohol, ammonia chemical precipitation, high-pressure hydrothermal synthesis, high-temperature solid-state sintering, inorganic salt sol-gel method, and freeze-drying, the preparation method of this invention has significant advantages, including simple operation and short reaction time. The obtained zinc-iron spinel single crystals exhibit superior physicochemical properties, including high purity, large size, and stable chemical performance. Most importantly, the synthesized zinc-iron spinel product has a high calcium content (4000-5000 ppm wt%) and a high water content (200-400 ppm), and the calcium and water contents can be completely controlled. The large particle size of zinc-iron spinel single crystals fully meets the sample requirements for high-temperature and high-pressure experiments on diamond pressure chambers, including simulations of the physical properties and spectroscopic characteristics of single crystal minerals under high temperature and pressure conditions, such as conductivity, synchrotron X-ray diffraction, confocal Raman spectroscopy, and vacuum Fourier transform infrared spectroscopy. This method provides important experimental sample support for measuring the physical property parameters of calcium-doped high-water-content zinc-iron spinel single crystals, especially for exploring the optimal orientation of the single crystal lattice and the anisotropy of the crystal axis under high pressure, thus breaking through the technical bottleneck of existing zinc-iron spinel single crystal synthesis. Detailed Implementation

[0052] A method for preparing calcium-doped, high-hydration zinc-iron spinel single crystals under high temperature and high pressure, comprising: using solid white fine amorphous basic zinc carbonate inorganic compound powder (purity: >99.99%), solid transparent reddish-brown flake-like iron(III) citrate crystals (purity: >99.99%), solid oxalic acid powder (purity: >99.99%), solid fine white calcium stearate powder (purity: >99.99%), solid zinc hydroxide powder (purity: >99%), solid slaked lime powder (purity: >99%), and liquid dilute nitric acid (concentration: 10%) as starting materials.

[0053] The initial material selected in this invention, high-purity solid basic zinc carbonate, is a white, fine, and amorphous inorganic compound powder. It is odorless and tasteless, readily soluble in dilute acids and sodium hydroxide, slightly soluble in ammonia, and insoluble in water and alcohol. It is mainly used in industrial production in fields such as latex, rayon production, skin protectants, pharmaceuticals, and feed additives. Solid basic zinc carbonate powder is chosen because of its stable properties and superior solubility in dilute acids, making it an excellent raw material for providing zinc in artificially synthesized zinc-iron spinel. The initial material selected in this invention, high-purity solid ferric citrate (III), is a transparent, reddish-brown, flaky crystalline inorganic compound, insoluble in ethanol but soluble in water and dilute acids. Ferric citrate (III), as a typical edible citrate, can be used in food science research as an iron fortifier, acidulant, and nutritional supplement; and in pharmaceutical research, it can be used in radioactive pharmaceuticals for monitoring iron metabolism abnormalities and hematopoietic function. Ferric citrate (III) crystals were chosen because of their excellent properties of readily decomposing in dilute acid solutions and strong chemical reactivity, making them an ideal raw material for providing iron in synthetic zinc-iron spinel. The initial material selected in this invention, high-purity solid calcium stearate, is a fine, white, fluffy, and smooth-feeling organic compound powder, almost insoluble in solvents such as water, acetone, and chloroform, and hygroscopic in air. Calcium stearate decomposes in acid solutions and has wide applications in the industrial production and preparation of plastics, paints, coatings, lubricants, and mold release agents. This invention selects fine, white solid calcium stearate powder because it dissolves in dilute nitric acid solutions, making it an excellent raw material for providing trace amounts of the alkaline earth metal calcium in synthetic zinc-iron spinel. The high-purity solid oxalic acid selected as the initial material in this invention is a chelating agent for metal substances. Its purpose is to leverage the significant impact of oxalic acid powder on the bioavailability of minerals, exhibiting a strong chelating effect. When oxalic acid combines with divalent zinc ions, it greatly reduces their solubility, thus forming a complex sol of divalent zinc ions in dilute nitric acid solution. Simultaneously, when oxalic acid combines with alkaline earth metal cations (calcium), due to its chelating effect, a soluble alkaline earth metal cation complex is formed, significantly enhancing the solubility of divalent calcium cations in acid solutions, allowing them to fully dissolve in dilute nitric acid solution. The high-purity solid zinc hydroxide selected as the initial material in this invention is a typical zinc-rich powdered aqueous substance. At 125°C, it undergoes a dehydration reaction to produce zinc oxide (ZnO), releasing a large amount of water. The high-purity solid slaked lime selected as the initial material in this invention is a typical calcium-rich powdered aqueous substance. At 580°C, slaked lime undergoes a dehydration reaction to produce quicklime (CaO), releasing a large amount of water.The dilute nitric acid (concentration: 10%) used as the initial material in this invention may cause residues of basic zinc carbonate powder, ferric citrate (III) crystals, calcium stearate powder, and oxalic acid powder if the nitric acid concentration is too low due to its limited solubility. If the nitric acid concentration is too high, its oxidizing properties will be enhanced, causing the basic zinc carbonate in the sample to undergo a rapid oxidation reaction or decompose directly, producing dense smoke, which may pose certain dangers to the preparation process.

[0054] Step 1: Open the chemical fume hood, select a standard 100 ml volumetric flask, accurately weigh out 60 ml of 10% dilute nitric acid, place a glass pipette in a 500 ml notched beaker, and carefully transfer all the liquid dilute nitric acid into the beaker along the pipette. The notched beaker is chosen as the reaction vessel mainly because it is not completely sealed after the glass watch glass is covered, and the generated gas can easily evaporate in the fume hood.

[0055] Step 2: On a 10-microgram high-precision analytical balance, accurately weigh out 5.0 grams of high-purity white fine and amorphous basic zinc carbonate inorganic compound powder, carefully add it to a notched beaker containing a 10% concentration of dilute nitric acid solution, and place a magnetic stirring rotor inside.

[0056] Step 3: Using a glass petri dish, cover the notched mouth of the beaker containing the dilute nitric acid solution of solid basic zinc carbonate powder and place it on a high-temperature magnetic stirring plate in a fume hood. In order to fully dissolve the initial material, solid basic zinc carbonate powder, in the dilute nitric acid solution, and at the same time allow it to undergo hydrolysis and acidification reactions, the reaction conditions are room temperature, 700 rpm, and 72 hours.

[0057] Step 4: According to the stoichiometric ratio of zinc-iron spinel (Zn,Ca)Fe2O4, accurately weigh 22.3074 grams of high-purity solid iron citrate (III) crystals and 120 milligrams of high-purity solid calcium stearate powder on a high-precision analytical balance, and carefully add them separately to a dilute nitric acid solution containing basic zinc carbonate.

[0058] Step 5: Place the dilute nitric acid solution containing solid basic zinc carbonate powder, solid iron (III) citrate crystals, and solid calcium stearate powder into a beaker, cover it with a glass watch glass to ensure that the gas generated by the reaction evaporates through the beaker's notch, and to prevent the dilute nitric acid solution of the initial materials in the beaker from splashing out during high-speed stirring, which could cause danger and affect the accuracy of zinc-iron spinel single crystal synthesis.

[0059] Step 6: Place the sealed beaker containing the initial dilute nitric acid mixture and the magnetic stirring rotor on a high-temperature magnetic stirring plate inside a fume hood. Under conditions of room temperature, a stirring speed of 800 rpm, and a stirring time of 48 hours, the initial solid basic zinc carbonate powder, solid ferric citrate (III) crystals, and solid calcium stearate powder are completely dissolved in the dilute nitric acid mixture without any residue. Simultaneously, this process facilitates the evaporation of volatile substances such as NH3·H2O, CH4, C2H2, CO2, CO, and H2 within the fume hood.

[0060] Step 7: Accurately weigh 2 grams of high-purity solid oxalic acid powder on a high-precision analytical balance. Add the high-purity oxalic acid powder, which is an important metal chelating agent, to a dilute nitric acid solution containing solid basic zinc carbonate powder, solid ferric citrate (III) crystals, and solid calcium stearate powder. The purpose is that oxalic acid powder has a great influence on the bioavailability of minerals and has a very strong chelating effect. When oxalic acid combines with divalent zinc ions, it can greatly reduce their solubility, thereby forming a complex sol of divalent zinc ions in the dilute nitric acid solution. At the same time, when oxalic acid combines with alkaline earth metal divalent cation calcium, due to its chelating effect, a soluble alkaline earth metal divalent cation calcium complex is formed. The solubility of the divalent calcium metal cation in the acid solution will be significantly enhanced, allowing it to fully dissolve in the dilute nitric acid solution.

[0061] Step 8: Place the notched beaker of the mixture back on the high-temperature magnetic stirring plate in the fume hood, cover it with a glass watch glass, and set the conditions of the high-temperature magnetic stirring plate to 80°C, 1000 rpm and stirring time for 36 hours, so that all the initial reagents form a uniform sol under the combined action of the mixed solution of dilute nitric acid and oxalic acid.

[0062] Step 9: Remove the glass watch glass from the beaker, and increase the temperature of the high-temperature magnetic stirring plate to 110°C until the mixed solution in the entire notched beaker is completely evaporated.

[0063] Step 10: Remove the magnetic stirring rotor from the notched beaker on the high-temperature magnetic stirring plate and clean all the powder sample adhering to its surface into the beaker. Carefully remove all 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 a certain concentration of carbon monoxide and carbon dioxide during the high-temperature calcination process, thereby controlling the oxygen fugacity of the zinc-iron spinel sample inside the graphite crucible, and ultimately constraining the valence state of the variable-valence metal cation iron in the zinc-iron spinel sample.

[0064] Step 11: Place the graphite crucible containing the mixed powder into a muffle furnace under normal pressure and high temperature conditions, raise the temperature to 1100℃ at a relatively slow heating rate of 300℃ / hour, and hold at that temperature for 5 hours. The relatively slow high-temperature calcination rate and the longer holding time are intended to better control the oxygen atmosphere within the graphite sample chamber, and to better remove residual nitric acid, oxalic acid, and other organic matter from the mixed powder.

[0065] Step 12: Cool the mixed sample powder in the graphite crucible in the muffle furnace to room temperature at a cooling rate of 200℃ / hour. Compared with the heating rate, choosing a slower cooling rate makes it easier to form a honeycomb-like loose sample powder. Carefully remove the mixed sample powder.

[0066] Step 13: Place the honeycomb-shaped loose zinc-iron spinel 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.

[0067] Step 14: Mix the uniform and fine-grained zinc-iron spinel powder sample and, using a high-precision tungsten carbide die with dimensions Φ10.0mm×10.0mm on a stainless steel tablet press, cold press it into three sample discs with dimensions Φ10.0mm×3.0mm. Carefully stack the three cold-pressed sample discs vertically together at the bottom of a graphite crucible.

[0068] Step 15: On the wall of the graphite crucible containing three stacked samples, drill two symmetrical circular holes with a diameter of 1.0 mm using a high-speed electric drill. Carefully thread a 0.5 mm platinum-rhodium alloy wire through the two symmetrical circular holes in the graphite crucible wall, suspending it in the center of the high-temperature oxygen atmosphere furnace. Fix the two ends of the platinum-rhodium wire connecting the graphite crucible to a vertical four-hole alumina tube with a diameter of 0.6 mm. The four-hole alumina tube has an outer diameter of 5.0 mm and a length of 40 cm. The upper end of the four-hole alumina tube is fixed in the center of a circular lid that allows for easy insertion and removal of the furnace body.

[0069] Step 16: Place a 3-liter stainless steel container filled with deionized purified cold water on the side of the high-temperature oxygen atmosphere furnace. The purpose is to allow the graphite crucible containing the sample to be pulled directly out of the high-temperature oxygen atmosphere furnace at extremely high temperatures and quickly immersed in the 3-liter stainless steel container of deionized water for rapid cooling. The main purpose is to prevent the variable valence element iron from being oxidized / reduced again during the slow cooling of the furnace, to achieve rapid quenching of the sample, and to completely preserve the glassy state of the zinc-iron spinel sample.

[0070] Step 17: At the very top of the high-temperature oxygen atmosphere furnace, it is interconnected with argon inert gas cylinders and adjustable proportions of carbon monoxide and carbon dioxide cylinders. The amount of gas introduced into the sample chamber is controlled by a pressure gauge. During the high-temperature calcination of the sample, each gas can be switched and adjusted at any time via valves. This invention uses argon inert gas to provide an absolutely reducing oxygen atmosphere environment when the furnace temperature is below 800°C.

[0071] This invention employs a precisely proportioned mixture of carbon monoxide and carbon dioxide to effectively control oxygen fugacity during high-temperature calcination of samples when the furnace temperature exceeds 800°C. If argon inert gas is continuously introduced when the furnace temperature exceeds 800°C, it will lead to over-reduction within the sample chamber, potentially causing variable-valence element iron to be reduced sequentially to metallic iron. Therefore, at temperatures above 800°C, we use a precisely proportioned mixture of carbon monoxide and carbon dioxide to control the oxygen fugacity of the sample within the high-temperature oxygen atmosphere furnace chamber. The reaction principle is as follows: It can effectively adjust the partial pressure of oxygen in the sample chamber, thereby controlling the valence state of iron, a variable-valence metal element, in calcium-doped high-hydration zinc-iron spinel single crystals.

[0072] The maximum rated temperature of the high-temperature oxygen atmosphere furnace body is 1800℃. Turn on the circulating cooling water of the high-temperature oxygen atmosphere furnace to reduce the temperature of the furnace body, preventing the overall furnace temperature from becoming too high and potentially causing carbon monoxide and carbon dioxide leaks, which could lead to a hazard. Activate the highly sensitive monitoring alarms for argon, carbon monoxide, and carbon dioxide concentrations to prevent gas leaks during the high-temperature calcination process in the oxygen atmosphere furnace and ensure operator safety.

[0073] Step 18: Open the argon inert gas valve and rotate the pointer button controlled by the gas pressure gauge to continuously purge the sample chamber for 30 minutes. This is to properly expel excess air from the sample chamber. Under the protection of argon inert gas, calcine the sample to 800℃ at a heating rate of 400℃ / hour.

[0074] Step 19: After the furnace body temperature reaches 800℃, quickly switch the carbon monoxide cylinder and carbon dioxide gas control valve, and rotate the pointer button controlled by the gas pressure gauge to make the volume ratio of carbon monoxide and carbon dioxide in the sample oxygen atmosphere furnace reach 4:1. The purpose is that during the high-temperature calcination process, the mixed gas of carbon monoxide and carbon dioxide with this volume ratio can effectively regulate the oxygen fugacity in the sample chamber.

[0075] Step 20: After the mixed gas flow of carbon monoxide and carbon dioxide (volume ratio 4:1) controlling the oxygen fugacity in the sample chamber reaches stability (this step requires approximately 3–5 minutes), the temperature of the sample chamber inside the furnace is then raised to 1420°C at a heating rate of 200°C / hour, and held at that temperature for 15 minutes to melt it into a glassy state of zinc-iron spinel. During the heating process in the high-temperature oxygen atmosphere furnace, two completely different heating rates of 400°C / hour and 200°C / hour were used for the sample chamber in different temperature ranges: from room temperature to 800°C and from 800°C to 1420°C. This invention, by applying a relatively slow heating rate as the temperature of the sample chamber inside the high-temperature oxygen atmosphere furnace rises, will be more conducive to the formation of strong ionic bonds such as Zn–O, Fe–O, and Ca–O in calcium-doped zinc-iron spinel; it will achieve more precise temperature control of the sample chamber inside the high-temperature oxygen atmosphere furnace; and it will completely avoid the problem of excessively high temperatures in local areas of the furnace due to unbalanced heat transfer in the sample chamber, which could easily damage the heating element of the oxygen atmosphere furnace, among other objectives.

[0076] The purpose of the high-temperature calcination process using a mixture of carbon monoxide and carbon dioxide to control the oxygen atmosphere is to provide a purer glassy state of zinc-iron spinel for the synthesis of large-particle calcium-doped high-hydration zinc-iron spinel single crystals in this invention; the high-temperature calcination under oxygen atmosphere conditions can better control the valence state of the variable-valence metal element iron in the product; and the relatively high calcination temperature of 1420℃ can ensure that any small amount of volatiles, nitric acid, oxalic acid, organic matter, and other substances that may remain after high-temperature calcination in the muffle furnace and affect sample preparation have all been completely volatilized.

[0077] A relatively short calcination time of 15 minutes at a constant temperature is used because zinc-iron spinel powder melts rapidly at temperatures above 1370℃. If the calcination time is too short, some initial powder residue may remain in the melted zinc-iron spinel, severely affecting the chemical composition of the prepared zinc-iron spinel sample. A short calcination time also hinders the sufficient chemical diffusion of zinc, iron, and calcium ions, and impedes the formation of stable chemical bonds in the strong ionic bonds (Zn–O, Fe–O, Ca–O, etc.) within the zinc-iron spinel. Furthermore, a short calcination time can lead to uneven distribution of doped calcium elements, such as stratification and differentiation, severely impacting the preparation results. A short calcination time also reduces the density of the product, making it difficult to form highly dense zinc-iron spinel glass. However, a calcination time exceeding 15 minutes may result in excessive melting, causing the zinc-iron spinel sample to adhere firmly to the graphite crucible wall, making it difficult to clean and increasing sample preparation costs.

[0078] Step 21: After the sample has been calcined at a constant temperature of 1420℃ 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 double-deionized purified cold water to rapidly quench it into zinc-iron spinel glass. The purpose of rapid quenching is to preserve the glassy zinc-iron spinel sample with uniform composition at high temperature.

[0079] Step 22: Carefully remove the quenched glassy zinc-iron spinel sample from the graphite crucible and grind it thoroughly in a corundum mortar to form a fine-grained and homogeneous powder. Place the glassy zinc-iron spinel powder in a vacuum drying oven at 200°C and dry for 12 hours.

[0080] Step 23: On a cold isostatic press, zinc-iron spinel glass powder is cold-pressed into cylindrical zinc-iron spinel samples with a diameter of 4.0 mm × 10.0 mm using a high-precision tungsten carbide mold of Φ4.0 mm × 4.0 mm.

[0081] To obtain zinc-iron spinel with high water content, we used zinc hydroxide powder (molecular formula: Zn(OH)2) and quicklime powder (molecular formula: Ca(OH)2) in a weight ratio of 4:1 as the water source. The choice of a mixture of zinc hydroxide and quicklime as the water source was based on the following considerations: First, both zinc hydroxide and quicklime are typical hydrated substances with relatively low dehydration temperatures. The dehydration temperature of zinc hydroxide is 125℃, while that of quicklime is 580℃. Therefore, these dehydration temperatures are achievable within the lower temperature range of the high-temperature, high-pressure process for preparing calcium-doped zinc-iron spinel single crystals. This ensures that the calcium-doped zinc-iron spinel single crystals are in a sufficiently long aqueous environment to guarantee the full diffusion of lattice water in the sample. The formation of lattice sites is crucial; secondly, both zinc hydroxide and quicklime are zinc-rich and calcium-rich substances, respectively, which can effectively control the zinc and calcium activities during the preparation of calcium-doped and high-water-content zinc-iron spinel single crystals within the sample chamber under high temperature and high pressure conditions; finally, the zinc hydroxide and quicklime in a 4:1 weight ratio placed at both ends of the sample, along with the dehydration products of the water source material combination—zinc oxide (ZnO) and quicklime (CaO)—do not chemically react with the sample, ensuring the purity of the calcium-doped and high-water-content zinc-iron spinel single crystal samples. Furthermore, by adjusting the weight ratio of zinc hydroxide and quicklime providing the water source and the corresponding height of the water source plate, the water content in the calcium-doped high-water-content zinc-iron spinel single crystal sample can be adjusted.

[0082] Step 24: On a cold isostatic press, zinc hydroxide powder and quicklime powder are cold-pressed in a 4:1 weight ratio using a high-precision tungsten carbide mold with a diameter of 4.0mm × 10.0mm to form two water source sheets with a diameter of 4.0mm × 0.1mm (height).

[0083] Step 25: Seal the cylindrical zinc-iron spinel sample (size: Φ4.0mm (diameter) × 4.0mm (height)) and two water source plates (size: Φ4.0mm (diameter) × 0.1mm (height)) sequentially within a double-capsule experimental sample chamber consisting of an inner sleeve – a graphite tube (size: Φ4.4mm (outer diameter) × 4.4mm (height), wall thickness 0.2mm) and an outer sleeve – a gold-palladium alloy tube (size: Φ4.6mm (outer diameter) × 4.6mm (height), wall thickness 0.1mm). In this invention, the calcium-doped zinc-iron spinel sample is placed in the exact center of the inner graphite sleeve; while the two water source plates, containing zinc hydroxide and quicklime in a 4:1 weight ratio, are placed at the symmetrical ends of the inner graphite sleeve close to the sample.

[0084] The inner sleeve of the double-capsule sample chamber of this invention uses graphite as the sealing material. Its main purpose is to control the oxygen fugacity values ​​of carbon monoxide and carbon dioxide within the sample chamber, thereby ultimately constraining the valence state of the variable-valence metal element iron in the zinc-iron spinel sample.

[0085] The outer sleeve of the double-capsule sample chamber of this invention uses a gold-palladium alloy as the sealing material. The main purposes are: firstly, the gold-palladium alloy seal isolates the sample from the exchange of substances or elements with other pressure-transmitting materials, effectively preventing sample contamination during the preparation of zinc-iron spinel samples under high temperature and high pressure; secondly, the gold-palladium alloy seal effectively prevents water from escaping from the sample tube during the preparation of zinc-iron spinel samples under high temperature and high pressure; finally, the double-capsule sample chamber, composed of a graphite tube and a gold-palladium alloy tube, creates a more sealed oxygen atmosphere environment, better controlling the oxygen fugacity within the sample chamber, thereby more effectively constraining the valence state of the variable-valence metal element iron in the zinc-iron spinel sample.

[0086] Step 26: Zinc-iron spinel is one of the important zinc- and iron-rich oxide minerals in the lower crust and upper mantle regions of Earth and other terrestrial planets. To realistically simulate the growth environment of zinc-iron spinel in the lower crust of Earth and other terrestrial planets, and to invert the temperature and pressure conditions for the stable existence of zinc-iron spinel mineral phases, a double-capsule sample chamber composed of graphite tubes and gold-palladium alloy tubes was placed on a typical 6–8 type multi-faceted top large-cavity high-temperature and high-pressure equipment of Kawai-1000t in the laboratory. The pressure and temperature were set to 0.5 GPa / hour and 10℃ / minute, respectively. Hot pressing sintering was carried out under the conditions of 3.0 GPa and 1100℃, respectively, and the reaction time was constant temperature and pressure for 72 hours.

[0087] The preparation process using a high pressure of 3.0 GPa and a sintering temperature of 1100 °C selected in this invention is designed entirely based on the physicochemical properties of zinc-iron spinel itself. The specific objectives are as follows: First, the preparation process under high temperature and high pressure conditions, a relatively slow rate of pressure and temperature increase, and a relatively long isothermal and isobaric reaction time can completely guarantee the mineral phase transformation from the initial zinc-iron spinel glass phase powder to the zinc-iron spinel crystalline phase, and the final product, the zinc-iron spinel mineral phase, can exist stably under these temperature and pressure conditions. Second, the preparation process under high temperature and high pressure conditions, a relatively slow rate of pressure and temperature increase, and a relatively long isothermal and isobaric reaction time significantly increases the self-diffusion and chemical diffusion coefficients of metal cations such as zinc ions, iron ions, and calcium ions, thereby achieving isomorphic substitution of zinc ions by calcium ions in the zinc-iron spinel crystal, with complete reaction and no residual free calcium element, thus forming a perfect calcium-doped zinc-iron spinel single crystal sample of alkaline earth metal element. Third, the preparation process under high temperature and high pressure conditions, a relatively slow rate of pressure and temperature increase, and a relatively long isothermal and isobaric reaction time can completely ensure the formation of stable chemical bonds such as Zn–O, Fe–O, and Ca–O, thereby avoiding the formation of calcium dopant in zinc. The process involves the formation of stratification and differentiation within the iron spinel, resulting in a uniform equiaxed crystal system of calcium-doped zinc-iron spinel single crystals. Secondly, the high-temperature, high-pressure conditions, relatively slow pressure and temperature increase rates, and long isothermal and isobaric reaction times allow for the dehydration reaction of a 4:1 weight ratio of zinc hydroxide and quicklime, producing a large amount of water. The final dehydration product is a mixed oxide of zinc oxide and quicklime. Simultaneously, this allows water to fully diffuse within the calcium-doped zinc-iron spinel single crystals in the sample chamber. The process of high temperature and high pressure, relatively slow pressure and temperature increase rate, and long isothermal and isobaric reaction time ensures that the calcium element is more evenly distributed in the final zinc-iron spinel product. At the same time, it increases the density, strength and particle size of the product, thus preparing a calcium-doped zinc-iron spinel single crystal sample with large particles of equiaxed crystal system and high water content, which has superior physicochemical properties such as uniform element distribution, high mechanical strength and high density.

[0088] Temperature was precisely calibrated using two sets of high-temperature resistant tungsten-rhenium thermocouples. Tungsten-rhenium thermocouples offer advantages such as good temperature-potential linearity, reliable thermal stability, and low cost, enabling temperature calibration within a range of 0-2300℃. They are widely used in ultra-high temperature calibration in fields such as high-pressure mineral physics experiments, advanced metallurgical industries, high-temperature electronic thermoelectric system structural engineering, space vehicles, and nuclear reactors. Each set of tungsten-rhenium thermocouples is composed of two different tungsten-rhenium alloys with the following chemical composition: W... 95％ Re 5％ and W 74％ Re 26％Tungsten-rhenium thermocouple wires of different materials, each with a diameter of 0.1 mm, were joined together at one end and twisted into a spiral shape using a vise. The other ends of the wires were connected to the positive and negative terminals of a high-power welding regulated DC power supply. The output current control knob of the power supply was adjusted to apply a large current to the wires, completely immersing the twisted tungsten-rhenium thermocouple wires in a saturated sodium chloride solution. The wires were melted and welded into spheres, and the oxide layer on the surface of the spherical thermocouple wires was removed. Using the same technique, two sets of tungsten-rhenium thermocouples were prepared, and each set was symmetrically placed at the upper and lower ends of a double-capsule sample chamber composed of a graphite tube and a gold-palladium alloy tube. This invention employs a double thermocouple consisting of tungsten and rhenium placed at the top and bottom of the sample chamber. This technology enables precise temperature calibration within the sample chamber and accurately indicates the temperature gradient at both ends of the sample chamber, ensuring that the zinc-iron spinel sample remains in a stable isothermal zone during the synthesis process.

[0089] Step 27: After maintaining a constant temperature and pressure for 72 hours at 3.0 GPa and 1100℃, the temperature inside the sample chamber is reduced from 1100℃ to 800℃ at a cooling rate of 3℃ / min, and held at that temperature for 1 hour. Then, the temperature inside the sample chamber is reduced from 800℃ to room temperature at a cooling rate of 5℃ / min. This step-wise cooling and the relatively slow constant-pressure cooling rate relative to the sample preparation heating rate (10℃ / min) further enhance the superior physicochemical properties of the calcium-doped zinc-iron spinel single crystal sample, which exhibits uniform calcium distribution, high mechanical strength, and high density. This completely avoids the uneven stress caused by excessively rapid cooling, which could lead to cracks and breakage in the zinc-iron spinel crystal. Furthermore, this preparation process is more conducive to the growth of large-particle zinc-iron spinel single crystals, thus enabling the preparation of large-particle zinc-iron spinel single crystal samples at the hundred-micron level.

[0090] Step 28: After the temperature inside the sample chamber drops to room temperature, the pressure inside the sample chamber is reduced from 3.0 GPa to atmospheric pressure at a depressurization rate of 0.5 GPa / hour. Furthermore, this invention provides a process for preparing calcium-doped high-hydration zinc-iron spinel single crystal samples via hot-pressing sintering. The preparation process is pure, without the introduction of any possible impurities from the sample itself or from high-pressure sample assembly.

[0091] Step 29: After the high-temperature, high-pressure preparation reaction is completed, the sample is removed from the typical 6–8 type multi-faceted top-large cavity high-temperature, high-pressure equipment of the Kawai-1000t. Carefully remove the graphite tube and gold-palladium alloy tube from the double-capsule sample chamber containing the sample. Using a high-precision diamond wire cutter, the cylindrical sample is cut open from the center. Under a high-precision Olympus microscope at 20x magnification, zinc-iron spinel single crystals are selected.

[0092] The zinc-iron spinel single crystal obtained in this invention is a single phase without any other impurity phases; electron probe microanalysis (EPMA) results show that the molecular formula of the obtained zinc-iron spinel single crystal is ZnFe2O4; multifunctional ion mass spectrometry (ICP-MS) results show that the calcium content in the obtained zinc-iron spinel single crystal is 4341 ppm wt%; vacuum Fourier transform infrared spectroscopy (FT-IR) results show that the water content of the obtained zinc-iron spinel single crystal sample is 385 ppm wt, indicating a high water content.

[0093] The calcium-doped, high-hydration zinc-iron spinel single crystal obtained in this invention is cubic with space group Fd3m (no. 227) and lattice parameters of [missing information]. α=β=γ=90°, unit cell volume is The average particle size is 175 micrometers, and the maximum particle size is 548 micrometers.

[0094] The calcium-doped high-hydration zinc-iron spinel single crystals obtained by this invention exhibit superior properties such as high purity, large particle size, stable chemical properties, and high mechanical strength. Most importantly, they possess a high calcium content (4341 ppm wt%), and this calcium content can be completely controlled. By changing the amount of high-purity calcium stearate powder added as the initial substance from 110.5663 mg to 138.2079 mg, the corresponding calcium content in the obtained calcium-doped high-hydration zinc-iron spinel single crystal samples can be adjusted from 4000 ppm wt% to 5000 ppm wt%. Furthermore, by changing the weight ratio of zinc hydroxide powder and quicklime powder used as the water source, and adjusting the different heights of the two water source plates, the total water volume generated by the dehydration reaction of the water-containing substances in the double-capsule sample chamber composed of graphite tubes and gold-palladium alloy tubes can be controlled, ultimately achieving the adjustment of the water content in the zinc-iron spinel single crystals. The obtained calcium-doped high-hydration zinc-iron spinel single crystal samples can fully meet the needs of physical experimental simulation of minerals in the lower crust and upper mantle of Earth and other terrestrial planets under high temperature and high pressure conditions. It breaks through the existing technical bottleneck of zinc-iron spinel single crystal synthesis and provides important experimental sample support for the study of the optimal orientation of single crystal mineral lattices and crystal axis anisotropy in the lower crust and upper mantle of Earth and other terrestrial planets under high temperature and high pressure conditions.

Claims

1. A method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure, characterized in that: The method includes: preparing cylindrical zinc-iron spinel samples using solid basic zinc carbonate inorganic compound powder, solid transparent reddish-brown flake-shaped iron (III) citrate crystals, solid oxalic acid powder, solid calcium stearate powder, solid zinc hydroxide powder, solid slaked lime powder, and liquid dilute nitric acid as starting materials; preparing water source sheets using zinc hydroxide powder and slaked lime powder in a weight ratio of 4:1 as water sources; and sequentially sealing the cylindrical zinc-iron spinel samples and two water source sheets in an inner sleeve-graphite tube before conducting a high-temperature and high-pressure reaction to obtain high-water-content zinc-iron spinel single crystals; the preparation method of the cylindrical zinc-iron spinel samples includes: Step 1: Weigh out 60 ml of 10% dilute nitric acid and place the glass pipette into a 500 ml notched beaker. Step 2: Weigh out 5.0 grams of basic zinc carbonate inorganic compound powder and add it to a notched beaker, then place a magnetic stirring rotor inside. Step 3: Cover the notched beaker with a glass watch glass and react at 700 rpm for 72 hours at room temperature. Step 4: According to the stoichiometric ratio of zinc-iron spinel (Zn,Ca)Fe2O4, weigh out 22.3074 g of solid iron citrate (III) crystals and 120 mg of solid calcium stearate powder and add them to a dilute nitric acid solution containing basic zinc carbonate. Step 5: Cover the notched beaker with a glass watch glass. Step 6: Place the notched beaker on a high-temperature magnetic stirring plate and stir at 800 rpm for 48 hours at room temperature. Step 7: Weigh out 2 grams of solid oxalic acid powder and add it to the notched beaker; Step 8: Place the notched beaker on the high-temperature magnetic stirring plate in the fume hood, cover it with a glass watch glass, and stir at 80 °C and 1000 rpm for 36 hours. Step 9: Remove the glass watch glass from the beaker, and adjust the temperature of the high-temperature magnetic stirring plate to 110°C until the mixed solution in the entire notched beaker is completely evaporated. Step 10: Remove the magnetic stirring rotor from the notched beaker on the high-temperature magnetic stirring hot plate, remove all the mixed powder from the notched beaker, and place it in the graphite crucible; Step 11: Place the graphite crucible containing the mixed powder into a muffle furnace under normal pressure and high temperature conditions, raise the temperature to 1100 °C at a heating rate of 300 °C / hour, and hold the temperature for 5 hours. Step 12: Cool to room temperature at a rate of 200 °C / hour, and remove the mixture sample powder; Step 13: Place the mixed sample powder in a corundum mortar and grind for 1 hour; Step 14: Cold press the ground mixture sample powder into 3 sample discs, and place the 3 cold-pressed sample mixture discs vertically together at the bottom of the graphite crucible; Step 15: Suspend the graphite crucible containing the three stacked samples in the center of the high-temperature oxygen atmosphere furnace. Step 16: Place a stainless steel container filled with deionized pure cold water on the side of the high-temperature oxygen atmosphere furnace. Step 17: Connect the top of the high-temperature oxygen atmosphere furnace to an argon inert gas cylinder and a carbon monoxide and carbon dioxide cylinder with adjustable ratios. Step 18: Open the argon inert gas valve and continue to purge for 30 minutes. Under the protection of argon inert gas, calcine the sample to 800 °C at a heating rate of 400 °C / hour. Step 19: After the furnace body temperature reaches 800 °C, switch the carbon monoxide gas cylinder and carbon dioxide gas control valve to make the volume ratio of carbon monoxide and carbon dioxide passing through the sample oxygen atmosphere furnace reach 4:

1. Step 20: Increase the temperature of the sample chamber inside the furnace to 1420 °C at a heating rate of 200 °C / hour, and bake at a constant temperature for 15 minutes. Step 21: After the sample is calcined at a constant temperature of 1420 °C for 15 minutes, the graphite crucible containing the sample is pulled out of the furnace and directly immersed in a stainless steel container to quench into zinc-iron spinel glass. Step 22: Take the quenched glassy zinc-iron spinel sample out of the graphite crucible, grind it in a corundum mortar, and then place the glassy zinc-iron spinel powder in a vacuum drying oven at 200 °C for 12 hours. Step 23: Using a cold isostatic press, zinc-iron spinel glass powder is cold-pressed into cylindrical zinc-iron spinel samples with a diameter of Φ 4.0 mm × a height of 4.0 mm using a tungsten carbide mold.

2. The method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure according to claim 1, characterized in that: The following substances were prepared: basic zinc carbonate inorganic compound powder with a purity >99.99%; solid transparent reddish-brown flake-like ferric citrate (III) crystals with a purity >99.99%; solid oxalic acid powder with a purity >99.99%; solid fine white calcium stearate powder with a purity >99.99%; solid zinc hydroxide powder with a purity >99%; solid slaked lime powder with a purity >99%; and liquid dilute nitric acid with a concentration of 10%.

3. The method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure according to claim 1, characterized in that: The method for preparing water source sheets using zinc hydroxide powder and quicklime powder in a weight ratio of 4:1 is as follows: On a cold isostatic press, zinc hydroxide powder and quicklime powder are cold-pressed in a weight ratio of 4:1 using a tungsten carbide mold to form two water source sheets with a diameter of Φ 4.0 mm × a height of 0.1 mm.

4. The method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure according to claim 1, characterized in that: Methods for obtaining high-water-content zinc-iron spinel single crystals by sequentially sealing cylindrical zinc-iron spinel samples and two water source plates within an inner sleeve-graphite tube and then subjecting them to a high-temperature, high-pressure reaction include: Step 25: Seal the cylindrical zinc-iron spinel sample and two water source plates in sequence in a double-capsule experimental sample chamber with an inner casing of graphite tube and an outer casing of gold-palladium alloy tube. Step 26: Place the double-capsule sample chamber on a typical 6–8 type multi-faceted top large-cavity high-temperature and high-pressure equipment of Kawai 1000t in the laboratory. Set the pressure increase rate and the temperature increase rate to 0.5 GPa / hour and 10 °C / minute, respectively. Under the conditions of raising the pressure and temperature to 3.0 GPa and 1100 °C, respectively, hot pressing sintering is carried out, and the reaction time is constant temperature and pressure for 72 hours. Step 27: Reduce the temperature inside the sample chamber from 1100 °C to 800 °C at a cooling rate of 3 °C / min and hold the temperature for 1 hour; then reduce the temperature inside the sample chamber from 800 °C to room temperature at a cooling rate of 5 °C / min. Step 28: After the temperature inside the sample chamber drops to room temperature, reduce the pressure inside the sample chamber from 3.0 GPa to atmospheric pressure at a depressurization rate of 0.5 GPa / hour. Step 29: Remove the sample from the typical 6–8 type multi-faceted top large cavity high temperature and high pressure equipment of Kawai 1000t, remove the graphite tube and gold-palladium alloy tube of the double-cell sample chamber that encloses the sample, cut the cylindrical sample from the middle with a diamond wire cutter, and select the zinc-iron spinel single crystal under an Olympus microscope.

5. The method for preparing calcium-doped high-hydration zinc-iron spinel single crystals under high temperature and high pressure according to claim 1, characterized in that: During the high-temperature and high-pressure reaction, two sets of tungsten-rhenium thermocouples were used for temperature calibration; each set of tungsten-rhenium thermocouples was composed of two different tungsten-rhenium alloys with the chemical composition 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 the double-capsule sample chamber composed of graphite tubes and gold-palladium alloy tubes.