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Formula: Cu2OCl2
Oxyhalide
Specific gravity: 4.08 calculated
Colour: Black, bluish black, deep brown in transmitted light
Solubility: Partly disssolved in water, completely in warm dilute acids
Environments
Localities
At the type locality, Mt Vesuvius, Somma-Vesuvius Complex, Naples, Campania, Italy, melanothallite is platy
or scaly, to 1 cm in size, in fumeroles, sublimed on crater walls formed in 1868 and 1906. Associated minerals
include eriochalcite,
chalcocyanite, euchlorine
and dolerophanite
(HOM, Mindat).
At the Tolbachik volcano, Kamchatka Krai, Russia, melanothallite occurs in volcanic fissures from an eruption
in 1975-1976. Plates up to 1 cm across were found. The melanothallite alters rapidly in a few days to a green
material, and when heated in air it decomposes to tenorite at about
400oC. Associated minerals include euchlorine,
chalcocyanite, dolerophanite
and tenorite
(AM 68.852).
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