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Mineral formula: Ca2Al2Si3O10(OH)2

Crystal system: orthorhombic

Crystal habit: Uncommon crystals are blocky to thin tabular; aggregates are fanlike, reniform, globular, stalactitic, granular, compact.

Cleavage: good on {001}, poor on {110}

Fracture: irregular/uneven

Color: colorless, gray, yellow, yellow-green, or white

Luster: vitreous, pearly

Diaphaneity: translucent

Moh’s scale hardness: 6 – 6 ½

Streak: white

Specific gravity: 2.8 – 2.95

Named after: the Dutch Colonel, Hendrik von/van Prehn, who is credited with discovering the mineral in 1774 at the Cape of Good Hope in South Africa. 

Type locality:  Karoo dolerites, Cradock, Inxuba Yethemba Local Municipality, Chris Hani District Municipality, Eastern Cape, South Africa

Geological occurrence: A secondary mineral in mafic volcanic rocks, also a low-grade metamorphic mineral.