Nucleation, stability and change of hexagonal CaAl₂Si₂O₈ crystals in the solution of anorthite₇₀forsterite₁₀silica₂₀ wt% were investigated by high temperature in-situ observations, ex-situ runs in a furnace, and examinations of the quenched products.
Hexagonal CaAl₂Si₂O₈ was observed to crystallize dominantly in the supercooled liquid at temperatures from 1100 to 1000°C. The incubation time becomes shorter as the temperature decreases. At 1000°C, it nucleates within a few hours. At these temperatures anorthite rarely nucleates in spite of large ΔT (300–400°C), but does at 1250–1150°C (ΔT=150–250°C). Its nucleation, however, needs time more than several hours. The nucleation sequence is explainable with supercooling degree and interfacial free energy difference for each polymorph. It was also demonstrated that the hexagonal polymorph has a metastable liquidus at 1240°C, 160°C lower than that of anorthite. This difference is consistent with that in the An₁₀₀ composition. The crystals of hexagonal CaAl₂Si₂O₈ grow steadily below this metastable liquidus. This can exist permanently if anorthite is not present in the system. However, hexagonal CaAl₂Si₂O₈ changes easily to anorthite without dissolution once the latter starts crystallization and gets in contact with the former. The contact of the two phases triggers the change from hexagonal to triclinic CaAl₂Si₂O₈.

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The phase equilibrium studies of the PbS–Sb₂S₃ join were carried out by the evacuated glass tube method and DTA at temperatures above 400°C. In addition to the end-members of stibnite and galena, three minerals, zinkenite, robinsonite and boulangerite, and five synthetic phases D, E, F, H and I were found on this join, and their chemical compositions, thermal stabilities and crystallographic properties were determined.
Stibnite melts congruently at 546°C. Zinkenite has a composition of 9Pb·11Sb₂S₃ and melts incongruently to robinsonite and liquid at 546°C. A eutectic reaction between stibnite and zinkenite occurs at 516°C. Robinsonite having 4PbS·3Sb₂S₃ composition melts incongruently to phase D and liquid at 582°C. Phase D having a composition of 5PbS·3Sb₂S₃ is stable on the PbS–Sb₂S₃ join at temperatures from 510° to 590°C, at which it melts incongruently to phase E and liquid. Phase E with an ideal composition of 7PbS·4Sb₂S₃ is stable at temperatures from 584° to 603°C and melts incongruently to boulangerite and liquid at 603°C. Phase F with a composition of 2PbS·Sb₂S₃ is stable from 490° to 584°, at which it decomposes to phase E and boulangerite. Boulangerite with a composition of 5PbS·2Sb₂S₃ melts incongruently to phase H and liquid at 640°C. Phase H having a composition of 3PbS·Sb₂S₃ is stable only at a temperature range from 625° to 647°, and incongruently melts to gelena and liquid at 647°C. Phase I has a composition of 16PbS·5Sb₂S₃, stable from 610° to 636°C, at which it decomposes to phase H and galena.

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The structures and physical properties of diopside, walstromite, wollastonite, parawollastonite, pseudowollastonite, wollastonite-II, and CaSiO₃ perovskite were simulated by computational models using energy minimization technique. The potential energies of these crystals are approximated by the sum of Coulomb, van der Waals, and repulsion terms between atoms. Required potential energy parameters of Ca and Ba were determined by fitting the calculated crystal structures of diopside and walstromite, and calculated bulk modulus of diopside to the observed ones. Using the obtained potential parameters of Ca and Ba, together with those of Mg, Si, and O derived from Matsui (1988), crystal structures of wollastonite, parawollastonite, pseudowollastonite, wollastonite-II, and CaSiO₃ perovskite were reproduced well. The elastic constants of diopside and the bulk modulus of CaSiO₃ perovskite were also simulated, and the values are in agreement with experimental data. These potential parameters were applied to predicting the bulk moduli and linear compressibilities of interatomic bonds of these crystals. The results of these calculations show that bulk modulus and compressibilities of M(Mg,Ca,Ba)-O bonds are related with crystal structures.

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