Prehnite

Prehnite
Prehnite (light) with epidote (dark)
General
Category	Silicate mineral
Formula (repeating unit)	Ca2Al(AlSi3O10)(OH)2
Strunz classification	9.DP.20 (Inosilicate transitional to phyllosilicate)
Dana classification	72.1.3.1 (Phyllosilicate)
Crystal system	Orthorhombic
Crystal class	Pyramidal (mm2) (same H-M symbol)
Space group	P2cm
Identification
Color	Colorless to gray to yellow, yellow-green or white
Crystal habit	Globular, reniform to stalactitic
Twinning	Fine lamellar
Cleavage	Distinct on [001]
Tenacity	Brittle
Mohs scale hardness	6 - 6.5
Luster	Vitreous - pearly
Diaphaneity	Semi-transparent to translucent
Specific gravity	2.8 - 2.95
Optical properties	Biaxial (+)
Refractive index	nα = 1.611 - 1.632 nβ = 1.615 - 1.642 nγ = 1.632 - 1.665
Birefringence	δ = 0.021 - 0.033
Dispersion	weak r > v
Ultraviolet fluorescence	Fluorescent, Short UV=blue white mild peach, Long UV=yellow.
References

Prehnite is an inosilicate of calcium and aluminium with the formula: Ca₂Al(AlSi₃O₁₀)(OH)₂. Limited Fe³⁺ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most oftens forms as stalactitic or botryoidal aggregates, with only just the crests of small crystals showing any faces, which are almost always curved or composite. Very rarely will it form distinct, well-individualized crystals showing a square-like cross-section, including those found at the Jeffrey Mine in Asbestos, Quebec, Canada. Prehnite is brittle with an uneven fracture and a vitreous to pearly luster. Its hardness is 6-6.5, its specific gravity is 2.80-2.90 and its color varies from light green to yellow, but also colorless, blue, pink or white. In April 2000, rare orange prehnite was discovered in the Kalahari Manganese Fields, South Africa. Prehnite is mostly translucent, and rarely transparent.

Though not a zeolite, prehnite is found associated with minerals such as datolite, calcite, apophyllite, stilbite, laumontite, heulandite etc. in veins and cavities of basaltic rocks, sometimes in granites, syenites, or gneisses. It is an indicator mineral of the prehnite-pumpellyite metamorphic facies.

It was first described in 1788 for an occurrence in the Karoo dolerites of Cradock, Eastern Cape Province, South Africa. It was named for Colonel Hendrik Von Prehn (1733–1785), commander of the military forces of the Dutch colony at the Cape of Good Hope from 1768 to 1780.