Formula: Ca6Al2(SO4)3(OH)12 · 26H2O

Colour: Colourless; colourless in transmitted light.

Hardness: 2 – 2½

Specific Gravity: 1.77

Crystal System: Trigonal

Member of: Ettringite group

Name: After the type locality at Ettringen, Rhineland-Palatinate, Germany.

Co-Type Localities: Mayener Feld, Mayen, Mayen-Koblenz, Rhineland-Palatinate – Germany;  Ettringer Feld, Ettringen, Vordereifel, Mayen-Koblenz, Rhineland-Palatinate – Germany

Although it is generally assumed that a solid solution series between ettringite and thaumasite exists, both species have different space groups and a possible discontinuity was identified. Ettringite can accept the replacement of 1/2 its Al by Si, while thaumasite tolerates little or no Al in its structure. The Al analogue of bentorite. The S (sulfate) analogue of siwaqaite.