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Formula: Mn2+4As5+Si3O12(OH)
Sorosilicate (Si2O7 groups) of manganese and
arsenic. Epitaxial on rhodonite
Crystal System: Monoclinic
Specific gravity: 3.84 measured, 3.829 calculated
Streak: White
Colour: Orange, brownish orange
Solubility: Insoluble in hydrochloric or sulphuric acid, slightly soluble in nitric acid
Common impurities: Ti,Al,V,Fe,Ca,Na,K
Environments:
Tiragalloite belongs to a rare class of compounds with arsenic As atoms replacing silicon Si atoms in
silicate anions, called arsenosilicates
(Mindat).
Localities
In the Ködnitz valley, Kals valley, Kals am Großglockner, Lienz District, Tyrol, Austria, tiragalloite has been found
in manganese-rich lenses in quartzitic chlorite
schist, probably derived from marine sediments. Associated minerals include
tephroite, pyroxmangite,
rhodonite, spessartine and
rhodochrosite
(HOM).
At the type locality, the Molinello mine, Ne, Genoa, Liguria, Italy, tiragalloite occurs as small grains enclosed in veinlets,
about one millimetre thick, cutting a black manganese ore made up mainly of quartz and
braunite, with traces of serpentine. The grains
are intimately associated with a number of other minerals, including quartz, manganese-bearing
calcite, albite,
parsettensite and medaite. Only fresh
unaltered boundaries between tiragalloite and the other minerals are observed, except against manganese-bearing
calcite, where a fine spongy rim shows alteration of tiragalloite grains
(AM 65.947-952).
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