Prehnite clusters originating from Morocco.

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 glass to pearly luster and a white streak color, and the cleavage is distinct according to the crystal plane [001]. Prehnite has an average density of 2.87 and a hardness of 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 is quite commonly found in hydrothermally altered rocks. Although it is not a zeolite, it is often found near 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 locality is Haslach, Harzburg, and Oberstein, Germany.