Gypsum (and anhydrite)

Typically occurs as euhedral to subhedral tabular crystals of simple habit (Figs 52 and 53) or platy cleavage fragments (Fig. 54), although it may occur as irregular anhedra. Large crystals may develop a lenticular habit through rounding of the prism {111} and pinacoid {103} (Siesser & Rogers, 1976). Cruciform interpenetration twins of diametral prisms may occur but are generally rare. Swallowtail contact twins of delicate tabular selenite may occur. Gypsum may show a wide variety of form and may occur as radiating clusters or rosettes of bladed and/or acicular crystals.