Prehnite Pseudomorph of laumontite and gyrolite Locality: Malad, Ward 38, Mumbai (Bombay), Mumbai District (Bombay District), Maharashtra, India size : 21x15c — Photo: Didier Descouens · CC BY-SA 4.0
Prehnite
A pale-green calcium-aluminium silicate forming botryoidal crusts — the first mineral named after a person (Colonel von Prehn).
Intro adapted from Wikipedia (CC BY-SA).
Prehnite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₂Al(Si₃Al)O₁₀(OH)₂.
It crystallizes in the orthorhombic crystal system (space group Pncm). It is a member of the Prehnite structural group.
Key properties: Mohs hardness 6–6.5; specific gravity 2.8–2.95; White streak; Vitreous, weak pearly on {001} luster; refractive index 1.633; optically biaxial.
IMA symbol Prh, grandfathered status — known before the IMA’s founding, named in 1789.
A secondary or hydrothermal mineral in veins and cavities in mafic volcanic rocks; less commonly in granite gneiss or syenite; a typical product of low-grade metamorphism.
It is found associated with Zeolites, datolite, pectolite, calcite, epidote, copper.
The name refers to Colonel Hendrik von Prehn (1733–1785), who discovered the mineral.