Prehnite

Prehnite is an inosilicate of calcium and aluminium with the formula: Ca₂Al(AlSi₃O₁₀)(OH)₂ with limited Fe³⁺ substitutes for aluminium in the structure.^[7] Prehnite crystallizes in the orthorhombic crystal system,^[7] and most often forms as stalactitic, botryoidal, reniform or globular aggregates,^[8] 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 6.5, its specific gravity is 2.80–2.95 and its color varies from light green to yellow, but also colorless,^[8] 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.

Prehnite
General
Category	Silicate mineral
Formula (repeating unit)	Ca2Al(AlSi3O10)(OH)2
IMA symbol	Prh[1]
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 to pearly
Streak	White
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	[2][3][4][5][6]

Though not a zeolite, prehnite is found associated with minerals such as datolite, calcite, apophyllite, epidote, stilbite, laumontite, and heulandite 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.^[4] 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.^[4]

It is used as a gemstone.^[9]

Extensive deposits of gem-quality prehnite occur in the basalt tableland surrounding Wave Hill Station in the central Northern Territory, of Australia.^[10]