An experimental mineralogical investigation of the 1 -atm thermal behavior of kalsilite, using tem, high-temperature X-ray powder diffraction, and high-temperature single-crystal diffraction, revealed a previously unreported unquenchable phase for KAlSio4. This phase, 3√3 kalsilite, appears stable between near 850 °C and less than 920 °C, above which characteristic superstructure diffraction effects weaken and fade into background. The high-temperature phase is a twinned pseudohexagonal phase with an orthorhombic unit cell characterized by a tripling of the room-temperature orthohexagonal dimension in the 5.16−Åhexagonal cell, tem evidence suggests that the room-temperature phase is twinned similarly to 3√3 kalsilite and might also be orthorhombic and pseudohexagonal. The transformation from kalsilite to 3√3 kalsilite is thermodynamically first order in character.

High-temperature powder-diffraction results for the synthetic polymorph of KAlSiO4 (Ol-KAlSiO4) showt hat the phaset ransition around 540°C is characterized by continuous and dramatic increase in the c/a ratio of the orthorhombic cell. Above Tc, the c/a ratio changes insignificantly up to 1000°C.

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