High-temperature induced dehydration, phase transition and exsolution in amicite: A single-crystal X-ray study

Abstract

Temperature dependent single-crystal X-ray data were collected on amicite K4Na4(Al8Si8O32)·11H2O from Kola Peninsula (Russia) in steps of 25°C from room temperature to 175°C and of 50°C up to 425°C. At room temperature amicite has space group I2 with a=10.2112(1), b=10.4154(1), c=9.8802(1)Å, β=88.458(1)°, V=1050.416(18) Å3. Its crystal structure is based on a Si–Al ordered tetrahedral framework of the GIS type with two systems of eight-membered channels running along the a and c axes. Extraframework K and Na cations are ordered at two fully occupied sites.Above 75°C amicite was found to partly dehydrate into two separate but coherently intergrown phases, both of space group I2/a, one K-rich ∼K8(Al8Si8O32) ·4H2O (at 75°C: a=10.038(2), b=9.6805(19), c=9.843(2) Å, β=89.93(3)°, V=956.5(3)Å3) and the other Na-rich ∼Na8(Al8Si8O32)·2H2O (at 75°C: a=9.759(2), b=8.9078(18), c=9.5270(19) Å, β=89.98(3)°, V=828.2(3)Å3).Upon further heating above 75°C the Na- and K-phases lost remaining H2O with only minor influence on the framework structure and became anhydrous at 175°C and 375°C, respectively. The two anhydrous phases persisted up to 425°C.Backscattered electron images of a heated crystal displayed lamellar intergrowth of the K- and Na-rich phases. Exposed to ambient humid conditions K- and Na-rich phases rehydrated and conjoined to the original one phase I2 structure.