Mineral Species: Mimetite and Calcite
Chemical Formula: Pb5(AsO4)3Cl
Chemical Name: Lead Chloroarsenate
Mineral Class: Arsenate
Mineral Notes: Mimetite forms in secondary, weathered zones of lead ore deposits. It is very similar in composition to Pyromorphite, the difference being the substitution of the Arsenate ion by Phosphate. Campyllite is a variety of Mimetite characterised by barrel-shaped orange-yellow crystals.
Description: The specimen consists of a pale grey matrix on which sit crusts made up of small, shiny crystals of Mimetite. These crystals have the expected brown colour and vitreous lustre of Mimetite, and under magnification, they appear to have good prismatic form. Some are even partially transparent. Surrounding the Mimetite are larger rhombohedral crystals of honey-coloured Calcite. A very attractive combination which displays very well under good illumination.
Location: Tsumeb Mine, Namibia.
