Mendipite

oxyhalide

limestone

Formula: Pb3O2Cl2 oxyhalide
Specific gravity: 7.24
Hardness: 2½ to 3
Streak: White
Colour: Colourless, white, grey, often tinged yellow, blue, red; nearly colourless in transmitted light
Solubility: Soluble in dilute nitric acid
Environments:

Sedimentary environments

The Mendips is a range of limestone hills, mostly in Somerset, England. This is the type locality for mendipite, which is one of the signature minerals of the manganese deposits there. Mendipite forms only when the supply of CO2 is restricted; if it is not, cerussite forms instead. This is why mendipite can only be found in the sealed environment of a cavity in the manganese oxides, isolated from the surrounding limestone which otherwise would be a source of abundant CO2.

The mendipite-hydrocerussite equilibrium reaction is
mendipite + CO2 + water ⇌ hydrocerussite + H+ + Cl-
Pb3O2Cl2 + 2CO2 + 2H2O ⇌ Pb3(CO3)2(OH)2 + 2H+ + 2Cl-

In addition to only forming at relatively high pH (alkaline), mendipite requires a steady supply of a moderately strong base to provide the hydroxyl ions (OH-) needed for the reaction.
3Pb2+ + 4OH- + 2Cl- = Pb3O2Cl2 (mendipite) + 2H2O

If there is an ample supply of litharge, mendipite can form according to the reaction
litharge + Cl- + water = mendipite + (OH)-
3PbO + 2Cl- + H2O = Pb3O2Cl2 + 2(OH)-

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