ETTRINGITE (Ca6[Al(OH)6]2(SO4)3.26H2O) occurs as a natural mineral, and is technically important as a hydration product of Portland and supersulphated cements and in its use, as satin white, for coating paper. It forms hexagonal, prismatic crystals which in synthetic material are often highly elongated. Bannister, Hey and Bernal1 obtained unit-cell data (hexagonal: a 11.26, c 21.48 A, space group P63/mmc, Z = 2). No crystal structure determination based on single crystal evidence has been reported, but Welin2 determined the structure of a related mineral, thaumasite (Ca6H4(SiO4)2(SO4)2(CO3)2.26H2O), which he found to be based on chains of Ca2+ and SiO44− ions with the remaining ions and water molecules in between.