gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1996.060.401.07 Mineralogy Twinning in an Orthorhombic Aluminate Sodalite, Ca8[Al12O24] (CrO4)2 Hassan Ishmael Department of Geology, Faculty of Science, University of Kuwait, P.O. Box 5969, Safat, Kuwait 13060 8 1996 60 401 617 622 1 6 1995 12 10 1995 Copyright © 1996, The Mineralogical Society 1996 Abstract

Transmission electron microscopy (TEM) was used to study the aluminate sodalite, Ca8[Al12O24](CrO4)2. This aluminate sodalite has four polymorphs: one cubic, one modulated, one tetragonal, and an orthorhombic cell that arise from ordering of [Ca4·CrO4]6+ clusters that occur in the cages of the structure. The transition temperatures are 432, 453, and 610 K. The cell parameters for the orthorhombic phase, with the cubic cell parameter of a = b = c = 9.222 Å, are 3a × 2b × c. Transformation twins are formed at 432 K during the transition from the tetragonal to the orthorhombic phase. The twin plane is {110} in the cubic cell, and {320} in the orthorhombic cell.

 Keywords aluminate sodalite twinning sodalite group nosean haüyne lazurite TEM superstructures