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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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