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)
H-M symbol: (mm2)
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 [1][2][3][4][5]

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.[3] 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.[3]

It is used as a gemstone.[6]

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

See also

References

  1. ↑ Mineralienatlas
  2. ↑ http://webmineral.com/data/Prehnite.shtml Webmineral data
  3. 1 2 3 http://www.mindat.org/min-3277.html Mindat
  4. ↑ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., ISBN 0-471-80580-7
  5. ↑ http://rruff.geo.arizona.edu/doclib/hom/prehnite.pdf Handbook of Mineralogy
  6. ↑ Tables of Gemstone Identification By Roger Dedeyne, Ivo Quintens, p.131
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