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Formula: (NH4)2(S6+O3S2-)
Thiosulphate [containing the anion (S6+O3S2-)]
Crystal system: Monoclinic
Specific gravity: 1.68 measured
Hardness: 2½
Colour: Colourless
Solubility: Dissolves instantly in water at room temperature
Environments
Burning bituminous shale
Sublimate
Hoperanchite is a new mineral, approved in 2024 and still pending publication in May 2025. It is the first
thiosulfate mineral that does not contain essential Pb
(Mindat).
Localities
At the type locality, the Hope Ranch, Santa Barbara County, California, USA, hoperanchite has been found as a
new (in 2024) combustion condensate mineral. Although new as a naturally occurring mineral, ammonium thiosulfate
is a a highly soluble salt well-known as a consumer product with uses as a photographic fixer salt and an alternative
to cyanide-based compounds in gold leaching, as well as having further
applications in chemical and biological research. Hoperanchite is, therefore, one of a very small number of
new minerals that can be purchased in bulk in its synthetic form at the time of its approval as a new mineral species.
Hoperanchite, ie natural ammonium thiosulfate, was found in burning landslide talus (a slope formed especially
by an accumulation of rock debris) composed of bituminous and/or kerogenous (kerogen is solid, insoluble organic
matter in sedimentary rocks) shales and
sandstones. Material exposed from the landslide is porous, and
exposure to oxygen resulted in rapid weathering of minerals such as pyrite
that have exothermic oxidation reactions. Thermal insulation resulted in the buildup of heat to the point of
sustainable combustion.
Hoperanchite formed in close association with sulphur,
mascagnite and koktaite at the
vent interface where hot gas mixed with much cooler air. This environment is analogous to the Huron shale fire in
Ohio where a landslide of an oil-rich shale unit led to the formation of
non-volcanic fumarole environments and the discovery of the new (in 2024) minerals
huizingite-(Al) and
carlsonite
(AM110.5.829-830).
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