Prehnite
Image: Didier Descouens (CC BY-SA 3.0)
Mineral NamePrehnite
Chemical FormulaCa₂Al(AlSi₃O₁₀)(OH)₂
Mohs Hardness6.5
Crystal Systemorthorhombic
Lustervitreous
Streak Colorwhite
Cleavagegood on {001}
Fractureuneven
Specific Gravity2.9
Colorspale green, yellowish-green
Transparencytranslucent
Type LocalityAure Valley, Hautes-Pyrénées, Midi-Pyrénées, France
Mineral GroupSilicates
Usesjewelry, collector
Associated Mineralszeolites, epidote, calcite, quartz
Rarityuncommon
Description
This specimen displays botryoidal aggregates of pale green, translucent prehnite crystals forming a bubbly crust on a rock matrix. The individual crystals exhibit a vitreous to pearly luster, creating a textured surface.
Geological Context
Prehnite typically forms in the vesicles and veins of mafic igneous rocks, such as basalt and gabbro, under low-grade metamorphic conditions. It is a characteristic mineral of the prehnite-pumpellyite metamorphic facies, often associated with hydrothermal alteration.
