Formula: CaMg(AsO4)F
Anhydrous arsenate containing hydroxyl
Specific gravity: 3.75 to 3.79
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

Hydrothermal environments

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

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

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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