Mimetite

Formula: Pb5(AsO4)3Cl arsenate
Specific gravity: 7.1
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, brown, orange, yellow, green, grey
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Environments:

Hydrothermal environments

Mimetite is a secondary mineral occurring in the oxidation zone of high temperature hydrothermal lead deposits that also bear arsenic-containing minerals. While pyromorphite usually occurs only in the uppermost zones, mimetite can also appear in deeper deposit regions

Alteration

Solubility of mimetite
Pb5(AsO4)3Cl (solid) + 6H+ (aqueous) ⇌ 5Pb2+ (aqueous) + 3H2AsO-4 (aqueous) + Cl- (aqueous)

cerussite and aqueous H2AsO4-, Cl- and H+ to mimetite and aqueous H2CO3
5PbCO3 + 3H2AsO4- + Cl- + 7H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
or
5PbCO3 + 3HAsO42- + Cl- + 4H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
cerussite and mimetite can co-exist only under basic conditions at rather high PCO2.

Common impurities: Ca,F,Cr,V