Formula: Ca₂Mg(AsO₄)₂ · 2H₂O

Species: Phosphates – Arsenates

Colour: White, colorless, translucent pale green (nickel-bearing), brownish pink (cobalt-bearing)

Lustre: Vitreous

Hardness: 5

Specific Gravity: 3.4-3.5

Crystal System: Triclinic

Name: Named after its discovery locality, the Talmessi Mine, Iran.

Talmessite is a hydrated calcium magnesium arsenate, often with significant amounts of cobalt or nickel. It was named in 1960 for the type locality, the Talmessi mine, Anarak district, Iran. It forms a series with β-Roselite, where cobalt replaces some of the magnesium, and with gaitite, where zinc replaces the magnesium. Talmessite is a rare secondary mineral formed typically in the oxidized zone of some hydrothermal mineral deposits, as an alteration product of realgar, orpiment, or Cu–Ni arsenides. Cobalt-rich varieties are found in the oxidised zone of cobalt arsenide deposits. All these minerals are members of the fairfieldite group. Talmessite is dimorphic with wendwilsonite (which is not a member of this group). Talmessite occurs as prismatic crystals to 3 mm, as radiating fibrous aggregates or as fine crystalline aggregates; it may also be stalactitic or in crusts. Pure talmessite is white or colourless, and colourless in transmitted light, but nickel-rich varieties are pale green and cobalt-rich varieties may be brownish, pink or the purple colour typical of many cobalt minerals. The streak is white and crystals are transparent to translucent with a vitreous lustre.