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Formula: K3Cu7Fe3+O4(AsO4)4
Anhydrous arsenate
Crystal System: Monoclinic
Specific gravity: 4.39 calculated
Hardness: 4
Streak: Dark green with an olive hue
Colour: Black
Common impurities: Zn
Environments
Melanarsite is a relatively new mineral, approved in 2014 and to date (January 2024) reported only from the
type locality.
Localities
At the type locality, the Arsenatnaya fumarole, Second scoria cone, Northern Breakthrough, Great Fissure eruption,
Tolbachik Volcanic field, Milkovsky District, Kamchatka Krai, Russia, specimens with melanarsite were collected
at a depth of about 0.5 m below the surface and temperature of 360 to 380°C. Melanarsite was found in
incrustations mainly consisting of arsenates, oxides and sulfates. It is
believed that the melanarsite was deposited directly from the gas phase as a volcanic sublimate at temperatures
not lower than 380°C. It occurs as well shaped or, more commonly, coarse crystals up to 0.1 mm, rarely up to 0.4 mm in
size, separate or combined in clusters up to 1 mm across, rarely forming interrupted incrustations, up to 1 cm × 1 cm in
area and up to 0.2 mm thick, covering basalt
scoria. The most typical minerals associated intimately with
melanarsite are dmisokolovite,
shchurovskyite,
bradaczekite, hematite,
arsenic-bearing orthoclase,
johillerite, arsmirandite,
aphthitalite, tenorite,
langbeinite, anhydrite,
tilasite and
fluorophlogopite. In some cases the melanarsite is associated
closely with hatertite or
pharmazincite
(MM 80.5.855-867).
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