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Prehnite Mineral Facts:
Chemical Formula:
Ca2Al(AlSi3O10)(OH)2
Prehnite is
a phyllosilicate of calcium and aluminum.
It is not a zeolite, but is often associated with them and is found in
the same environments.
Colors:
Yellowish,
colorless
or light green.
Its streak is a colorless.
Hardness:
6 to 6.5
Density: 2.9
Cleavage:
Distinct cleavage on
(001).
Crystallography: Orthorhombic
Prehnite is nearly always found in crystals, though it occurs also in
stalactitic and granular masses. Isolated individuals are rare, usually many
are grouped together into knotty or warty aggregates.
Luster:.
Vitreous to resinous
luster. It is transparent to translucent.
Optics:
(Refractive Index):
=
a= 1.616, b = 1.626, y = 1.649
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Yellow botryoidal Prehnite |
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Composition,
Structure and Associated Minerals:
Prehnite is a mineral of
secondary origin lining amygdaloidal
cavities in basalt, and similar environments. Prehnite also occurs as crystals implanted on the walls of
clefts in siliceous rocks, in the gas cavities in lavas, and in the gangue
of
certain ores, especially
copper ores. It is found also as pseudomorphs
after analcite, laumonite, and natrolite. In
all cases it is probably a secondary product. Associated with zeolites, datolite, pectolite,
calcite, etc. Occurs in the United States at Farmington, Connecticut;
Paterson and Bergen Hill, New Jersey; Somerville, Massachusetts;
Lake Superior copper district. Found also in various
European localities.
Identification
and Diagnostics
Before the blowpipe prehnite
fuses very readily with exfoliation
and bleaching. It melts
with
intumescence to a white bubbly glass, eventually forming a
yellowish enamel. At a high temperature it yields water
that is quite visible in a closed tube. Its powder is
strongly alkaline. It is partially decomposed by strong hydrochloric
acid with the production of pulverulent silica. After fusion it dissolves
readily in this acid yielding gelatinous silica.
Some green prehnite closely resembles
olivine in appearance, and the green
variety has been used to a small extent as a gemstone. It can be readily
distinguished from peridot by its lower specific gravity, and
its fusibility.
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Prehnite |
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Occurrence,
Localities and Origins:
Prehnite is named after Colonel
Prehn, who first found the mineral in South Africa.
Prehnite localities: Fine crystals come from veins at Harzburg, in Thuringia;
at Striegau and Jordansmuhl, Poland, and at Fassa and other places in
Tyrol.
Good specimens
occur at Bourg d'Oisans in Dauphine, France associated with feldspar,
axinite
and
epidote.
Good crystals are found also in the Campsie Hills in Scotland.
The mineral is abundant in veins with copper ore along the north shore of
Lake Superior and on Keweenaw Point, and it occurs also at Farmington,
Conn.; Bergen Hill, N. J.; and Cornwall, Penn.
Uses. Prehnite makes fine collector mineral specimens, and is also
occasionally used as gem stone. Some translucent forms are used for gem
carving and other pieces are made into beads. Both the yellows and green
tones of this mineral are attractive. The mineral known as chlorastrolite is probably an impure
prehnite. It is found on the beaches of Isle Royale and the north shore
of Lake Superior as little pebbles composed of stellar and radial bunches
of bluish green fibers. The pebbles were originally the fillings of gas
cavities in old lavas. They are polished and used, to a slight extent, as
gem stones.
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