Arsentsumebite (?) after mimetite

bayldonite (?); mimetite (?)

A group of free-standing prismatic crystals of mimetite completely encrusted - and perhaps completely replaced - by arsentsumebite (?). The pseudomorphs (to 19 mm) are sharp and terminated by a simple pinacoid {001}.

The replacing arsenate mineral in this specimen has not been analysed; studies of similar specimens have shown that these pseudomorphs commonly consist of arsentsumebite and / or bayldonite in varying proportions. Arsentsumebite (?) is believed to be the dominant replacing mineral in this example.

The specimen is from the collection of Sharon and Eugene Cisneros. It was formerly in the collections of John "Jack" Parnau and Lou Perloff.  There is no formal record of location in the mine but the composition and habit are typical of material from the upper levels of the first oxidation zone.