Template:POTD/2023-11-15
Prehnite is an inosilicate mineral of calcium and aluminium with the chemical formula Ca2Al(AlSi3O10)(OH)2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most often forms as stalactitic, botryoidal, reniform or globular aggregates. Prehnite is brittle with an uneven fracture and a vitreous to pearly luster. It has a hardness of 6.5 on the Mohs scale and its specific gravity is 2.80 to 2.95, while its color varies from light green to yellow, with some specimens also colorless. Prehnite is used as a gemstone, and was first described in 1788 for an occurrence in the Karoo dolerites of Cradock in the Eastern Cape of South Africa. It was named after Colonel Hendrik Von Prehn, the commander of the military forces of the Dutch colony at the Cape of Good Hope from 1768 to 1780. This prehnite crystal, measuring 4.0 cm × 3.5 cm × 2.0 cm (1.6 in × 1.4 in × 0.8 in), was found in Southbury, Connecticut, in the United States.Photograph credit: Ivar Leidus