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Formula: Mn2+5(AsO4)2(AsO3OH)2.10H2O
Valence: Mn2+5(As5+O4)2(As5+O3OH)2.10H2O
Hydrated arsenate,
ondrušite group,
lindackerite supergroup,
manganese-bearing mineral
Crystal system: Triclinic
Specific gravity: 3.05 measured, 3.00 calculated
Hardness: 3
Streak: White
Colour: Very faint rose-red colour. Isolated crystals appear almost colourless.
Luminescence: No fluorescence under UV
Environments
Sedimentary environments
Metamorphic environments
Localities
At the type locality, Falotta, Tinizong, Surses, Albula Region, Grisons, Switzerland, geigerite occurs in
cavities in radiolarites, formed by metamorphism of manganese oxide
ores. Associated minerals include tilasite,
sarkinite, manganberzeliite,
brandtite, grischunite and
berglagite.
Geigerite covers the rock in minute crystals, rarely exceeding 0.5 mm in length, on areas of the order of
1 cm2. The colour is very faint, and isolated single crystals appear almost colourless. The crystals are platy
and striated, and have one perfect cleavage. Geigerite also occurs as massive, fine-grained to fibrous aggregates up
to 1 cm in length. Some specimens have a milky translucent aspect, presumably owing to an alteration process. The mineral
is extremely brittle and quite soft
(AM 74.676-684, HOM).
Geigerite from Falotta - Image
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