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Calaverite Mineral Facts:
Chemical
Formula: AuTe2 (Usually
contains a little silver)
44% Gold by weight
Colors:
Silver-white
or bronzy yellow in color and has a yellow-gray or greenish gray
streak. Its surface is frequently covered with a yellow tarnish.
Hardness: 2
to 3
Hardness varies somewhat depending
on formation and impurities.
Density: 9.4
The density
is high because of the large gold content.
Cleavage:
The mineral
is brittle and without distinct cleavage.
Crystallography: Monoclinic
Calaverite crystallizes in the monoclinic system in crystals that are
elongated parallel to the orthoaxis and deeply striated in this
direction. Twinning is common and the resulting twins are very
complicated. Usually, however, the mineral occurs massive and granular.
Luster:.
Metallic luster.
Optics:
(Refractive Index): Opaque
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Composition,
Structure and Associated Minerals:
Calaverite
is a nearly pure gold telluride. However, it is usually intermixed with
small quantities of the
silver telluride. Calaverite is chiefly important as
an ore of gold. While occurring only in a few places, they are sufficiently
abundant at some of them to be the main ore mined at these locations.
Identification and Diagnostics
On charcoal
before the blowpipe the mineral fuses easily to a yellow globule of gold,
yielding at the same time the fumes of tellurium oxide. It dissolves in
concentrated H2SO4, producing a deep red solution (a positive test for
telluride ion). When treated with
HNO3
it
decomposes, leaving a rusty mass of spongy gold. The solution treated with
HC1 usually yields a slight precipitate of silver chloride. Distinguished
from sylvanite by the small amount of silver present and by its lack of a
cleavage. It is distinguished from
krennerite by its mode of crystallization.
Occurrence,
Localities and Origins:
The
mineral occurs in veins with the other tellurides associated with gold ores
in Calaveras Co., California in
the Stanislaus Mine,
where it was first
recognized. Calaverite and other associated gold and silver telluride
minerals are
important sources of silver and gold in
the mines at Nagyag, Transylvania, at Cripple Creek and in Boulder Co.,
Colo., and at Kalgoorlie, W. Australia.
It is believed to have been deposited by ascending magmatic water at
comparatively low temperatures. Other rare tellurides belonging to this
group are,
sylvanite, hessite (Ag2Te) and petzite (Au,Ag2Te),
krennerite, AuTe2 , and nagyagite, a sulpho-telluride of lead and gold.
Historically, calaverite has been as
important ore of
gold.
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