Malachite after azurite (w. arsentsumebite after azurite (?))

arsentsumebite; azurite; cerussite; mimetite (?); pyrolusite (?)

A matrix of massive Cu/Pb arsenates with accessory malachite and sparse relict azurite hosts broken epimorphs of arsentsumebite (EDS verified) after both tabular and prismatic crystals (to 15 mm) of a monoclinic precursor – probably azurite (?) (Exhibit 2).

An apparently later generation of azurite crystals has been replaced by malachite which is in turn locally encrusted by charcoal-grey pyrolusite (?) – an Mn oxide with minor Zn and Cu content EDS analysed) and etched microcrystals of cerussite. A single (8 mm) fan of mimetite crystals, extensively (possibly completely) replaced by Cu/Pb arsenates is present, apparently post-dating the malachite after azurite pseudomorphs. Equant crystals (to 2 mm) of colourless cerussite complete the paragenesis.

SEM/EDS analysis of Cu/Pb arsenates by Martin Števko (Slovak Academy of Sciences, Bratislava, Slovakia); EDS analysis of black oxide material by Frank Keutsch (Harvard University).

The specimen is from the A.P. Holden Collection which was bequeathed to Harvard University in 1913, thereby dating the specimen to the upper part of the first oxidation zone (i.e. 6 Level or above).