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