Tilasite

tilasite

pyrochroite

spessartine

braunite

Images

Formula: CaMg(AsO4)F
Anhydrous arsenate, tilasite group
Crystal System: Monoclinic
Specific gravity: 3.77 measured, 3.80 calculated
Hardness: 5
Colour: Light gray, violet-gray, pinkish red (Långban); olive-green, apple-green (India); colourless or pale green in transmitted light.
Solubility: Readily soluble in hydrochloric and nitric acid
Environments:

Metamorphic environments
Fumeroles

Tilasite occurs typically in metamorphosed manganese or zinc deposits containing arsenic.

Localities

At Kajlidongri, India, tilasite occurs with spessartine, braunite, baryte, and quartz (HOM).

At the Monte Cervandone area, Devero Alp, Baceno, Verbano-Cusio-Ossola Province, Piedmont, Italy, tilasite has been found in clefts at the Wannigletscher and farther up, near Cervandone’s peak, tilasite is seen as fibrous aggregates and as pointed prismatic crystals to 1 cm. The best specimens, however, with tilasite crystals up to 1.5 cm in size, have come from the Italian side of the mountain. Tilasite crystals when fresh are pink, red, violet or brown, and can be transparent, but with weathering they turn cloudy and red-brown to grey-brown. Common associations include chlorite, epidote, muscovite and adularian orthoclase (MinRec 56.3.319-320)>

At the type locality, Långban, Filipstad, Värmland, Sweden, tilasite occurs in dolomitic limestone, (Dana) associated with pyrochroite, pyroaurite, hematite, hausmannite, dixenite, calcite, berzeliite, manganberzeliite, baryte, manganoan diopside, svabite, bergslagite and allactite (Mindat).

At Sterling Hill, New Jersey, USA, tilasite is associated with friedelite, willemite and baryte (HOM).

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