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Formula: Pb2Cu(AsO4)(SO4)(OH)
Compound arsenate,
tsumebite group,
brackebuschite supergroup
Crystal System: Monoclinic
Specific gravity: 6.46 measured, 6.39 calculated
Hardness: 3½ (R&M 97.3.236-243)
Streak: Light green
Colour: Emerald-green, grass-green, apple-green, pale bluish green
Environments:
Arsentsumebite is a rare arsenate that occurs as aggregates of small,
distorted crystals with individuals seldom exceeding 2 mm, or as powdery crusts.
Localities
At the Cap Garonne Mine, Le Pradet, Toulon, Var, Provence-Alpes-Côte d'Azur, France, the country rock consists of
conglomerates and
sandstones, and the minerals present are mainly
secondary species derived from the breakdown of
tetrahedrite/tennantite and
galena.
Bariopharmacoalumite occurs as colourless to pale yellow
interpenetrating cubes associated with mansfieldite,
philipsbornite, beudantite,
carminite, duftite,
mimetite, scorodite,
olivenite, arsentsumebite and
lavendulan
(AM 98.279).
At the Clara mine, Germany, arsentsumebite occurs in a hydrothermal polymetallic
baryte-fluorite deposit
(Webmin, HOM).
At the type locality, the Tsumeb mine, Oshikoto Region, Namibia, arsentsumebite occurs in the oxidised zone of a
dolostone-hosted hydrothermal polymetallic ore deposit
(Webmin, HOM, Mindat). It was not uncommon in the upper portion of the first oxidation zone, but considerably
less common at greater depths, and apparently not present at all in the third oxidation zone.
The arsentsumebite occurs typically as crusts of small, distorted and closely intergrown crystals. In some
attractive specimens it forms a contrasting colour backdrop to crystals of species such as blue-black
azurite, cerussite or colourless to
white anglesite. Much less conspicuously, arsentsumebite is often a
constituent of the massive copper
arsenate–rich matrix that is characteristic of many specimens; in such cases
the arsentsumebite is intimately intergrown with species that include
bayldonite, cerussite,
conichalcite, duftite,
gartrellite, malachite and
smithsonite.
Arsentsumebite forms epimorphs after
mimetite, encountered mainly in the first oxidation zone. Typically, these
arsentsumebite pseudomorphs are microcrystalline and usually they
have an earthy lustre; rarely they exhibit coarse crystallinity making for very attractive specimens. Much less common
than the pseudomorphs after
mimetite, arsentsumebite (sometimes in combination with
bayldonite) was also found replacing
azurite. Extensive coatings (or perhaps partial replacements) of
arsentsumebite on cerussite have also been recorded.
In the deeper levels of the mine arsentsumebite inclusions in flattened
anglesite crystals have been found. Also a second-oxidation-zone specimen
exists featuring arsentsumebite encrusting blue (yes, blue!)
wulfenite crystals on contrasting white
anglesite.
Arsentsumebite also occurs here with beudantite, though apparently
only in the absence of carminite. A small second-oxidation-zone pocket
comprised coarse chocolate-brown crystals of pseudo-hexagonal beudantite
overgrowing framboidal aggregates of goethite on a matrix of partly oxidised
sulphides. The beudantite is decorated with spherical tufts of
duftite that are in turn supplanted by isolated crystals of spearmint-green
arsentsumebite
(R&M 97.3.236-243).
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