Tridymite

About: Tridymite is a research topic. Over the lifetime, 840 publications have been published within this topic receiving 14831 citations.

Krotite, CaAl2O4, a new refractory mineral from the NWA 1934 meteorite

Abstract: Krotite, CaAl_2O_4, occurs as the dominant phase in an unusual Ca-,Al-rich refractory inclusion from the NWA 1934 CV3 carbonaceous chondrite. Krotite occupies the central and mantle portions of the inclusion along with minor perovskite, gehlenite, hercynite, and Cl-bearing mayenite, and trace hexamolybdenum. A layered rim surrounds the krotite-bearing regions, consisting from inside to outside of grossite, mixed hibonite, and spinel, then gehlenite with an outermost layer composed of Al-rich diopside. Krotite was identified by XRD, SEM-EBSD, micro-Raman, and electron microprobe. The mean chemical composition determined by electron microprobe analysis of krotite is (wt%) Al_2O_3 63.50, CaO 35.73, sum 99.23, with an empirical formula calculated on the basis of 4 O atoms of Ca_(1.02)Al_(1.99)O_4. Single-crystal XRD reveals that krotite is monoclinic, P2_1/n; a = 8.6996(3), b = 8.0994(3), c = 15.217(1) A, β = 90.188(6), and Z = 12. It has a stuffed tridymite structure, which was refined from single-crystal data to R_1 = 0.0161 for 1014 F_o > 4σF reflections. Krotite is colorless and transparent with a vitreous luster and white streak. Mohs hardness is ~6½. The mineral is brittle, with a conchoidal fracture. The calculated density is 2.94 g/cm3. Krotite is biaxial (–), α = 1.608(2), β = 1.629(2), γ = 1.635(2) (white light), 2V_(meas) = 54.4(5)°, and 2V_(calc) = 55.6°. No dispersion was observed. The optical orientation is X = b; Y ≈ a; Z ≈ c. Pleochroism is colorless to very pale gray, X > Y = Z. Krotite is a low-pressure CaAl_2O_4 mineral, likely formed by condensation or crystallization from a melt in the solar nebula. This is the first reported occurrence of krotite in nature and it is one of the earliest minerals formed in the solar system.

An electron diffraction study of the polymorphs of siO2-tridymite

Abstract: The results of a detailed TEM study of the low frequency modes of distortion of high temperature SiO2-tridymite and their relationship to the extensive structural polymorphism of tridymite are presented. It is found that low energy modes of distortion of the ideal tridymite tetrahedral framework structure give rise to a strong and extremely characteristic diffuse intensity distribution (which can be broken into two component types) for the two highest temperature polymorphs of tridymite. Experimental results strongly suggest that this observed diffuse distribution is not a result of irreversible beam damage but is rather an intrinsic property of the ideal tridymite tetrahedral framework structure. The diffuse intensity distribution is closely related to the lower temperature polymorphs of tridymite — in particular, the primary modulation wave-vectors of these low temperature polymorphs always fall on the higher temperature diffuse distribution. The first type of diffuse distribution appears to result from coupled tetrahedral edge rotation of 〈110〉 columns of corner-connected SiO4 tetrahedra (uncorrelated from column to column as a result of the tetrahedral connectivity of the ideal tridymite framework structure). The real space structural origin of the second curved type of diffuse distribution, however, remains unclear.

Rigid unit modes in the high-temperature phase of SiO2 tridymite: calculations and electron diffraction

Abstract: Calculations of the rigid unit mode (RUM) spectrum of the high-temperature phase of SiO2 tridymite are used to explain the patterns of diffuse scattering seen in transmission electron microscopy experiments. These results show that RUM's can occur with wave vectors on curved surfaces in reciprocal space rather than being confined to symmetry points, lines or planes. The fact that the calculations reproduce the detail seen in the diffuse scattering provides a striking nontrivial confirmation of the validity of the rigid unit mode model.

Phase Equilibrium Studies of “Cu2O”–SiO2–Al2O3 System in Equilibrium with Metallic Copper

Abstract: Phase equilibria of the “Cu2O”–Al2O3–SiO2 system have been experimentally investigated at metallic copper saturation. High-temperature equilibration, rapid quenching, and electron probe X-ray microanalysis (EPMA) techniques have been used. Containerless equilibration technique has been developed to enable the phase equilibrium of this chemically reactive system to be investigated. The microstructures and compositions of all phases present in the quenched sample were measured accurately using EPMA. The isothermals between 1150°C and 1300°C have been determined in the “Cu2O”–Al2O3–SiO2 system at metallic copper saturation. The following primary phase fields were identified in the system: SiO2 (tridymite), Cu2O (cuprite), Cu2O.Al2O3 (delafossite), Al2O3 (corundum), and 3Al2O3·2SiO2 (mullite). The implications of the phase diagram on making of the copper aluminosilicate glass have been demonstrated.

Microstructural Evolution in Clay‐Based Ceramics II: Ternary and Quaternary Mixtures of Clay, Flux, and Quartz Filler

Abstract: The complex microstructural evolution in mixtures of kaolinite clay, quartz, nepheline syenite, and soda–lime–silica (SLS) glass has been revisited. Detailed descriptions of reactions leading to dense whiteware bodies backed by semi-quantitative X-ray diffraction analysis reveal that ternary mixtures containing nepheline syenite differ from those containing SLS glass, as new crystalline phases develop inside and at the interfaces among SLS glass particles and decomposed clay and quartz. In SLS glass-fluxed mixtures, tridymite formation was hindered and cristobalite formed at ≥750°C followed by wollastonite at ≥800°C. Albite and plagioclase formed from interaction between clay and molten SLS glass above 800°C. Wollastonite was not present ≥1100°C, leaving only cristobalite in the surrounding regions. Quartz partially dissolves at temperatures ≥1000°C after interacting with molten SLS glass. In quaternary mixtures containing 6.25 wt% SLS glass and 17.25 wt% nepheline syenite fluxes, formation of types II and III secondary mullite was more pronounced than in the fully SLS glass-fluxed mixture. The more fluid liquid from nepheline syenite enhances the growth kinetics of mullite. The body fired at the optimum firing temperature (1100°C) has a microstructure containing primary type I and secondary types II and III mullites, remnant cristobalite, plagioclase, and partially dissolved quartz embedded in the glassy phase. Providing a roadmap for microstructural design in clay-based systems may have significant commercial impact in the emerging technology of use of waste materials in clay-based ceramics.