Prehnite with tourmaline hearts from Mali, 50-80 mm.

The mineral prehnite is a calcium-aluminum silicate with the chemical formula Ca2Al2Si3O10(OH). The mineral belongs to the phyllosilicates. The transparent to translucent colorless, white, gray or yellow to yellow-green prehnite has a vitreous to pearly luster, a white streak color and the cleavage is clear along the crystal plane [001]. Prehnite has an average density of 2.87 and the hardness is 6 to 6.5. The crystal system is orthorhombic and the mineral is not radioactive. The mineral prehnite is named after the Dutch Colonel Hendrik Von Prehn (1733 - 1785), who discovered the mineral in South Africa. Prehnite was the first mineral to be named after a person. Prehnite is a mineral that occurs fairly commonly in hydrothermally weathered rocks. Although it is not a zeolite, it is often found in the vicinity of zeolites. It is found, among other places, in granites, syenites and gneisses. It is also an indicative mineral for the low-grade metamorphic facies prehnite-pumpellyite. The type location is Haslach, Harzburg and Oberstein, Germany.