Formula: Mn2+4As5+Si3O12(OH)
Sorosilicate (Si2O7 groups) of manganese and arsenic. Epitaxial on rhodonite
Specific gravity: 3.84
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

Metamorphic environments

Tiragalloite belongs to a rare class of compounds with arsenic As atoms replacing silicon Si atoms in silicate anions, called arsenosilicates (Mindat).


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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