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Formula: Ag4Hg5S5(I,Br)2Cl2
Sulphide, silver- and mercury-
bearing mineral
Crystal system: Orthorhombic
Specific gravity: 6.75 calculated
Hardness: 2
Streak: Reddish orange
Colour: Bright red
Environments
Localities
At Broken Hill, Broken Hill district, Yancowinna County, New South Wales, Australia, perroudite occurs within
massive white kaolinite as aggregates of prismatic microcrystals up to
0.03 mm long and as single crystals intimately associated with
iodargyrite, to form slightly photosensitive, earthy to waxy
reddish-orange grains and patches. Seams containing tiny crystals of native gold
also occur within the perroudite-bearing kaolinite, along with
relict euhedra of spessartine.
Extensive deposits of white compact kaolinite occurred in several mines at
Broken Hill. They were often richly impregnated with films, veins, and plates of
silver halides (iodargyrite and
bromian chlorargyrite), seams and films of
native silver, and patches of
At Broken Hill, the white kaolinite that hosts the
silver- and mercury- bearing
halides was locally derived by alteration of primary
aluminosilicates such as sillimanite and
feldspars. The original metamorphic rocks, mainly
granitic gneisses, were
mineralised, mainly with galena and
sphalerite. Release of sulphur,
silver and mercury during
sulphide alteration, probably influenced by halide-rich solutions, provided the necessary ingredients for
perroudite crystallisation. The source of the mercury is uncertain,
as primary sulphides carrying significant
mercury are not known within the ore bodies. However,
silver-rich tetrahedrite
inclusions within galena are common in the
primary ore, and this mineral may also be the source of the
mercury. The high iodine concentrations in the oxidised zone at Broken Hill
have not been satisfactorily explained, although it is possible that the presence of sea water has influenced
mineralisation during the long and complex history of development of the
secondary zone.
(AM 72.1251-1256).
Perroudite from Broken Hill - Image
At Coppin Pool, Tom Price, Ashburton Shire, Western Australia, a suite of
supergene minerals including
anglesite, cerussite,
phosgenite, covellite,
pyromorphite, cinnabar, and
at least six new (in 1987) species, has formed by weathering of a
galena-bearing quartz vein.
Perroudite occurs as coatings of prismatic crystals up to 0.05 mm long by 0.025 mm wide on the walls of small
cavities in a heterogeneous supergene assemblage. Crystals
tend to be fluted and frequently show hollow terminations. The presence of a number of chlorine-bearing
secondary minerals suggests that this element was present in
abundance in the groundwater that was responsible for alteration of the
primary sulphides. Perroudite,
cinnabar, and several new (in 1987)
mercury-silver sulphides indicate
locally rich, but so-far (in 1987) unidentified, primary
sources of mercury
(AM 72.1251-1256).
At the type locality, the Cap Garonne Mine, Le Pradet, Toulon, Var, Provence-Alpes-Côte d'Azur, France,
perroudite forms fibrous, tufted aggregates of crystals up to 0.07 mm long by 0.02 mm wide within cavities in
sandstones and
conglomerates.
Secondary copper
minerals such as olivenite,
brochantite, cyanotrichite
and parnauite, as well as
mercury- and silver- bearing
tennantite, also occur in these sedimentary rocks.
Perroudite crystals from Cap Garonne may be contact-twinned. The crystallisation following decomposition of
mercury- and silver- bearing
tennantite by halide-bearing solutions, probably of marine origin.
(AM 72.1251-1256).
Associated minerals include anglesite,
cerussite, cinnabar,
covellite, gold,
iodargyrite, kaolinite,
phosgenite, pyromorphite
and tennantite subgroup minerals
(Mindat).
Perroudite from Cap Garonne - Image
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