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Formula: [Ag9CuS4][(Ag,Cu)6(Sb,As)2S7]
sulpharsenite,
pearceite-polybasite group, forms a series with
pearceite, antimony- and
arsenic- bearing mineral
Crystal System: Monoclinic
Specific gravity: 6.1 measured, 6.36 calculated
Hardness: 2½ to 3
Streak: Black
Colour: Black with dark ruby red internal reflections
Common impurities: As,Fe
Environments:
Polybasite occurs in silver veins of low to moderate temperature of formation, associated with
pyrargyrite, tetrahedrite,
stephanite, other silver
sulphosalts,
acanthite,
gold, quartz,
calcite, dolomite and
baryte (HOM).
Localities
In the Imiter district, Morocco, polybasite occurs on a quartz-rich
vein matrix
in a late silver assemblage,
associated with proustite. It is best developed in the
dolomite cavities. Sometimes crystals of
imiterite rest
on polybasite crystals, and polybasite sometimes forms a substrate for
acanthite crystals
(MinRec 42.2.126-127).
At the Caribou mine, Boulder county, Colorado, USA, polybasite has been found partially replaced by
stephanite (R&M 87.4.313).
At the Smuggler mine, Boulder county, Colorado, USA, a specimen has been found with crystals of polybasite
scattered across a plate of
milky quartz crystals. The polybasite crystals are partially coated
with and altered
to pseudomorphs
of silver and acanthite after
the high temperature form of silver sulphide known as argentite
(R&M 87.4.323).
At Silver Plume, Clear Creek county, Colorado, USA, polybasite twins have been found scattered across
partially resorbed crystals
of pseudomorphs
of acanthite after the high temperature form of silver sulphide known as
argentite
(R&M 87.4.311).
At the Eldorado Mine, Ouray county, Colorado, USA, polybasite has been found with
pyrargyrite as
small intergrown masses with base-metal sulphides. It also occurred as microcrystals associated with
chalcopyrite and pyrargyrite
(R&M 84.5.424).
At the Red Mountain district, Ouray county, Colorado, USA, polybasite has been found partially coated with and
replaced
by pseudomorphs of
silver and acanthite after the high
temperature form of
silver sulphide known as argentite (R&M 87.4.326).
At the Hermosa Mining District, Sierra County, New Mexico, USA, low temperature
lead-zinc-silver
mineralisation characterises the district. The ore occurs in fissure veins and replacement bodies in
limestone and dolomitic
limestone. The principal hypogene
minerals are galena, sphalerite,
chalcopyrite and argentite. Associated
minerals are pyrite, polybasite and
tetrahedrite. The gangue consists mainly of
talc, with associated calcite,
quartz, baryte, and
clay minerals. Supergene minerals
include covellite, native silver,
limonite, carbonates of lead,
zinc and copper,
silver halides, and a little vanadinite,
descloizite and pyromorphite.
Talc, which occurs both as vein filling and in wall-rock alteration, is thought to have been
formed by the action of silica-bearing hypogene solutions. Little transportation
appears to have taken place during the early stages of oxidation, as evidenced by the intimate association of
hypogene and supergene
silver minerals. The mineralisation is believed to have been brought about by alkaline
hydrothermal solutions that may be connected with the sources of volcanic activity in the area. An unusual feature of the deposits is
the occurrence of talc as the chief gangue mineral
(EG 49.7.759–778).
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