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Staurolite
Mineral Facts:
Formula:
(Fe,Mg,Zn)2Al9(Si,Al)4O20(OH)4
Staurolite is a
nesosilicate mineral
, etc.
Colors:
Reddish-brown, brownish-black,
sometimes yellowish-brown.
The streak is
colorless or greyish.
Hardness:
6
Density:
7 to 7.5
Cleavage:
The cleavage is
distinct parallel to (010).
Crystallography: Orthorhombic
Habit prismatic, showing
usually a combination of prism with large angle, brachipinacoid, base and
macrodome. Cruciform twins are very common. They are of two types, (1) in
which the two individuals cross at nearly 90 (see photo), and (2) in which
they cross at nearly 60 degrees.
Sometimes the
twinning is repeated, giving rise to trillings.
Luster:.
Subvitreous to resinous luster.
It is slightly
translucent in fresh crystals, ranging
translucent to opaque,
but usually is
opaque.
Crystals usually have dull, rough surfaces.
In very thin
pieces it is pleochroic in hyacinth red and golden yellow tints.
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Staurolite Crosses |
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Composition, Structure and
Associated Minerals:
Staurolite is
an accessory mineral in metamorphic rocks; in crystalline schists, clay
slates, and sometimes in gneisses.
The mineral
occurs principally in mica schist and other schistose rocks where it is the
result of a high grade of regional or contact metamorphism. Because of its
method of occurrence it frequently contains numerous mineral inclusions,
among them
garnet
and mica.
Often associated with garnet, cyanite, sillimanite and
tourmaline.
Staurolite
weathers fairly readily into micaceous minerals, such as
chlorite
and muscovite..
Identification
and Diagnostics
Before the blowpipe staurolite is infusible, unless it contains significant
amounts of manganese, in which case it fuses to a black magnetic glass.
Infusible. Insoluble in the standard acids, though it is slightly
attacked by sulfuric acid.
On intense ignition in closed
tube yields a little water. Often very impure and mixed with mica and other
minerals.
Staurolite is
distinguished from other minerals by its
characteristic
style of
crystallization,
crystal twins,
infusibility
and hardness.
Localities
Staurolite occurs as a
mineral of metamorphic origin in clay slates, mica schists, gneisses, which
have undergone high grade forces of heat and pressure.
Its composition
is variable and not well fixed.
The name is derived from a
Greek word stauros, meaning cross, and lithos, meaning stone; in allusion to
its cruciform twins. Notable localities for its occurrence are Monte
Campini, Switzerland; in Brittany; Minas Geraes, Brazil;
Good crystals
of staurolite are found in the schists at Mount Campione, Switzerland; in
the Zillerthal, Tyrol; at Aschaffenburg, in Bavaria; at various places in
Brittany, France; and in the United States, at
Franconia and Lisbon, New
Hampshire; Chesterfield, Massachusetts; Fannin County, Georgia;
in Patrick Co., Virginia, and in Fannin Co., North Carolina.
Uses.
Twins of staurolite are used, to a slight extent, as jewelry. Cruciform
specimens from Patrick County, Virginia, are mounted and worn as charms
under the name of "Fairy Stones."
Occasionally a transparent
stone from Brazil is cut as a faceted gem.
Staurolite is a
mineral that is interesting from the fact that it frequently forms twinned
crystals that resemble a cross in shape, and which consequently, during the
Middle Ages, was held in great veneration.
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