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Formula: PdHg
Alloy of palladium and mercury
Crystal System: Tetragonal
Specific gravity: 14.88 measured, 15.09 calculated
Hardness: 3½
Streak: Silver-white
Colour: Silver-white
Luminescence: Not fluorescent under UV
Solubility: Soluble in nitric acid (AM 13.491)
Common impurities: Cu
Environments
Plutonic igneous environments
Placer deposits
Potarite occurs in chromitite and
dunite ultramafics, and
in placers for gold and diamonds.
Associated minerals include platinum,
palladium, gold,
palladium-mercury compounds,
pentlandite, chalcopyrite,
pyrrhotite and millerite
(HOM).
Localities
At the Córrego Bom Sucesso placers, Serro, Minas Gerais, Brazil,
platinum–palladium nuggets up
to 1 mm in size have a botryoidal habit. A typical composite arborescent nugget comprises a broad irregular core
region of massive gold-bearing potarite, or cavity space with relict
potarite enclosed by a narrow zone of platiniferous
palladium. Other nuggets comprise an arborescent to dendritic core of
gold-bearing potarite, a broad internal zone of either pure
platinum or palladian
platinum and a narrow rim of
platinum.
It is suggested that the Bom Sucesso nuggets resulted from high-level episodic hydrothermal alteration of
mafic and ultramafic rocks
within the drainage basin, with the remobilised platinum and
palladium precipitated in open spaces in the enclosing
metaquartzites
(CM 40.341-355).
At the type locality, the Kaietur Falls, Potaro River, Kangaruma District, Potaro-Siparuni Region, Guyana,
potarite occurs as placers of white metallic nuggets, usually very rounded due to the mineral's low hardness.
Associated minerals include diamond,
palladium and platinum
(Mindat).
At Inatsumiyama, Nichinan, Hino District, Tottori Prefecture, Japan,
palladium-rich
platinum group minerals have been found in sulphide-bearing
dunites and
harzburgites from the
ultramafic complex. In the
dunite, potarite is the most abundant
platinum group mineral but other associated
platinum group minerals include
stibiopalladinite and rare
sperrylite, and palladium-rich
alloys. A palladium telluride
has been found in the harzburgites. The
platinum group minerals are usually enclosed by
pentlandite-heazlewoodite
composite grains and, rarely, by altered chromium-rich
spinel. These minerals are characteristic of an
ultramafic assemblage but here they are accompanied by ubiquitous
galena and minor sphalerite,
that are not usually associated with such assemblages. The ultramafic
part of this complex has been thermally metamorphosed
(olivine-talc zone) within the
contact aureole of an adjacent
granite. The
platinum group minerals, sulphides and altered
spinel are all intergrown with
antigorite and/or chlorite,
indicating a metamorphic overprint on the primary igneous
mineralogy. It is possible that potarite and the other
platinum group minerals formed at temperatures of up to 650oC
(MM 63.369-377).
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