Minerals ~ Prehnite
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Sliced Prehnite fragment with calcite core, 9.0 cm diameter.
Photo Credit: Yaxx
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.
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.
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.
It is used as a gemstone.
Extensive deposits of gem quality prehnite occur in the basalt
tableland surrounding Wave Hill Station in the central Northern
Territory, of Australia
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