Formula: Ca2Mg(AsO4)2.2H2O
Hydrated normal arsenate
Specific gravity: 3.57
Hardness: 5
Colour: White, colorless, translucent pale green (nickelian), brownish pink (cobaltian)

Hydrothermal environments

Talmessite is a rare secondary species typically formed in the oxidised zone of some hydrothermal mineral deposits. It is an alteration product of realgar, orpiment or copper-nickel arsenides, associated with gaitite, erythrite, annabergite, picropharmacolite, pharmacolite, austinite, fluorite, baryte, aragonite, calcite and dolomite.

At Luceram, Ales-Maritimes, France, talmessite is associated with baryte.

At the type locality, the Talmessi Mine, Esfahan Province , Iran, talmessite occurs in the oxidation zone asssociated with aragonite and dolomite.

At Bou Azzer, Morocco, talmessite is quite widespread, although it is rare worldwide. Here it occurs inter-grown with roselite-beta on a dolomite matrix associated with wendwilsonite.

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