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Formula: Mn2+6As5+Si5O18(OH)
Sorosilicate (Si2O7 groups), manganese- and
arsenic- bearing mineral
Crystal System: Monoclinic
Specific gravity: 3.772 calculated for the empirical formula
Streak: White
Colour: Orange-reddish
Environments
Metamorphic environments
Hydrothermal environments
Arsenmedaite is a relatively new mineral, approved in 2016.
Localities
At the type locality, the Molinello Mine, Ne, Genoa, Liguria, Italy, arsenmedaite was collected in small
quartz veinlets, a few mm in thickness, cutting the
braunite ore bodies hosted by Jurassic (201 to 145 million years ago)
metamorphosed cherts, which in turn overlay a
mafic-ultramafic
ophiolitic sequence.
Arsenmedaite was identified in a few specimens collected in the mine. It occurs as orange-reddish prismatic
crystals, up to 200 microns in length, with a vitreous lustre and a white streak. It is a late-stage mineral,
crystallising from arsenic- and
manganese- rich hydrothermal fluids that circulated through a network of
fractures at relatively low temperatures.
Arsenmedaite has been found associated with braunite,
quartz, calcite,
arsenic-rich medaite,
calcian rhodocrosite (a mixture of manganese-bearing
calcite, rhodochrosite and
rhodonite), talc and
ganophyllite
(EJM 31.1.117-126).
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