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Formula: Ca2Mg(AsO4)2.2H2O
Hydrated arsenate,
fairfieldite group
Crystal System: Triclinic
Specific gravity: 3.421 measured, 3.53 calculated
Hardness: 5
Colour: White, colourless, translucent pale green (nickeloan), brownish pink (cobaltoan)
Environments:
Talmessite is a rare secondary arsenate
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
(HOM).
Localities
At Luceram, Ales-Maritimes, France, talmessite is associated with
baryte
(Dana).
At the type locality, the Talmessi Mine, Esfahan Province , Iran, talmessite occurs in the oxidation zone
asssociated with aragonite and
dolomite
(AM45.1315, Dana).
At Bou Azzer, Morocco, talmessite is quite widespread, although it is rare worldwide. Here it
occurs inter-grown with anorthoroselite on a
dolomite matrix
associated with wendwilsonite
(Minrec 38.5).
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