What is "talmessite"

n. (context mineralogy English) A triclinic-pinacoidal mineral containing arsenic, calcium, hydrogen, magnesium, and oxygen.

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. All these minerals are members of the fairfieldite group. Talmessite is dimorphic with wendwilsonite (which is not a member of this group).