Mimetite, Pb5(AsO4)3Cl (Lead Chloroarsenate), 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 small orange-yellow rounded crystals.

Description: A relatively small piece, but containing a very unusual crystal habit of Mimetite. Rather than the classic barrel-shaped crystal this specimen consists of two large rounded botryoidal masses, together with three smaller globules. All are dark brown and have an earthy luster.

Location: Mexico Mine, Caldbeck Fells, England.