Images
Formula: Ag2AsS2
Sulphosalt, silver-bearing mineral
Crystal system: Monoclinic
Specific gravity: 5.62 calculated
Hardness: 1 to 1½
Streak: Dark brown
Colour: Grey
Solubility: Soluble in nitric acid
Environments
Localities
At the Trojická vein, Daniel adit level, Geschieber vein, Svornost Mine, Jáchymov, Karlovy Vary District, Karlovy
Vary Region, Czech Republic, the ore veins cut a complex of medium-grade metasedimentary rocks in an envelope of
granite plutons. The complex geochemistry of
primary mineralisation led to an unusually rich
supergene zone, with occurrences of numerous minerals formed
during post-mining processes.
Seven ore stages were distinguished within the Jáchymov ore field:
tin-tungsten
sulphoarsenide, ore-free quartz,
carbonate–uraninite, arsenide,
arsenic-sulphide, sulphide and post-ore stage.
The Trojická vein is one of the typical constituents of the oldest
tin-tungsten sulphoarsenide stage.
This stage is related to the auto-metamorphism of younger granite,
which underlies the whole district.
The dervellite was found in a vein of native arsenic. The vein is up
to 2 cm thick and locally brecciated, enclosing tiny fragments of
mica schists. The oldest mineral
in the association is skeletal or fine-grained native silver in aggregates
up to 5 mm in size, sometimes naturally etched from the native arsenic.
Arsenic also encloses rare small cubes of violet
fluorite. Rare cavities host thick pyramidal dark red
proustite crystals up to 2 mm in size, very rarely associated with
dervillite. Black prismatic dervillite crystals with strong metallic lustre up to 0.5 mm in size occur
only rarely. Dervillite replaces proustite along with usually
fine-grained native bismuth, which further replaces both dervillite
and proustite.
Supergene
arsenolite forms up to 2 mm large octahedra or their aggregates and
covers the rock fragments containing native arsenic. It also sometimes
fills fractures
(MM 89.257–265).
Dervillite from the Svornost Mine -
Image
At the type locality, the Gabe Gottes Mine, Sankt Jakob vein, Neuenberg, Sainte-Marie-aux-Mines, Colmar-Ribeauvillé,
Haut-Rhin, Grand Est, France, a museum specimen containing dervillite has been found from a cavity in the
cobalt - nickel -
iron - arsenic deposit. Associated
minerals include arsenic,
rammelsbergite,
safflorite, proustite,
calcite and quartz
At the Lengenbach Quarry, Fäld, Binn, Goms, Valais, Switzerland, dervillite occurs as very rare crystals up
to 200 μm across on jordanite
(MM 77.8.3105-3112).
Dervillite from the Lengenbach Quarry -
Image
Back to Minerals