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Formula: Ba0.5Al4[(AsO4)3(OH)4].4H2O
Hydrated arsenate,
pharmacoalumite Group,
pharmacosiderite supergroup,
barium-bearing mineral
Crystal System: Isometric
Specific gravity: 2.580 calculated for the empirical formula
Hardness: 3½
Streak: White
Colour: Colourless to pale yellow
Environments
Bariopharmacoalumite is a secondary mineral coating
fractures and cavities in
tetrahedrite-tennantite-bearing
rocks undergoing near-surface weathering.
(HOM).
Localities
At La Grande Mine, Marqueza, Arqueros Ag mining district, La Serena, Elqui Province, Coquimbo, Chile, the local geology
is dominated by andesitic
breccias consisting of bands of finely crystalline siliceous and
calcareous rock, alternating with bands of more coarsely crystalline calcareous rock, and it is within the latter that
most of the mineralisation is found.
The main gangue mineral is baryte,
which is found in large quantities on the outer boundaries of the mineralised veins. Other
gangue materials include quartz and
jasper. Near the surface, oxidised mineralisation can still be observed in
the form of secondary
copper minerals including carbonates
(malachite and azurite), silicates
(chrysocolla), sulphates
(brochantite) and arsenates
(cornubite and olivenite).
Secondary lead minerals
including mimetite and
pyromorphite have also been reported.
Bariopharmacoalumite occurs as zones within
bariopharmacosiderite crystals.
The bariopharmacoalumite-bariopharmacosiderite crystals
are associated with quartz and
chalcedony on a
limonite-goethite matrix, which
occurs close to the siliceous-calcareous strata.
The crystals range in colour from yellow to orange to light brown, and in size from 0.1 to 0.5 mm across. The crystals
are opaque, have an orange streak and a dull lustre
(MM 75.1.135-144).
Bariopharmacoalumite from La Grande Mine -
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At the type locality, Cap Garonne Mine, Le Pradet, Toulon, Var, Provence-Alpes-Côte d'Azur, France, mineralisation
occurs in sub-horizontal veins of conglomerates and
sandstones. The minerals present are mainly
secondary species deriving from the breakdown of
tetrahedrite-tennantite and
galena, usually developing in fractures or more rarely in cavities within the
country rock (such as perroudite/capgaronnite/iltisite
and ktenasite/guarinoite/theresemagnanite).
The type specimen of bariopharmacoalumite was collected from a long fracture in the conglomerate that extends
vertically from the lead-bearing levels at the top of the mineralised vein, down
to copper-bearing levels at the bottom. Several
aluminium-bearing minerals are observed in this fracture, including
mansfieldite and
philipsbornite. Other minerals present include
beudantite, carminite,
duftite, mimetite and
scorodite in the Fe/Pb area, and
olivenite and lavendulan in the
Cu area. Bariopharmacoalumite has also been found at other places in the Cap Garonne mines:
(1) In the North mine, as small (0.3 mm) white cubes covering fractures of the
conglomerates, where it is associated with
olivenite, arsentsumebite
and philipsbornite.
(2) In the South mine, bariopharmacoalumite was collected as isolated white cubes (to 0.5 mm) in fractures
containing zeunerite and
mansfieldite.
(3) Also in the South mine, in two other occurrences, bariopharmacoalumit is found with
lavendulan, mansfieldite and
olivenite.
Another bariopharmacoalumite-bearing specimen from the Cap Garonne mine from the Natural History Museum, London,
is a large 15 cm x 15 cm piece of fracture surface showing rich encrustations of pale yellow bariopharmacoalumite
cubes associated with mansfieldite, sky-blue
arsenogorceixite and
olivenite.
Bariopharmacoalumite typically occurs as radial growths of colourless to pale yellow interpenetrating cubes up
to several millimetres across. Individual cubes are generally less than 100 microns in size and some exhibit stepped
growth features. Some cubes are elongated, giving a rectangular outline
(MM 75.1.135-144).
Bariopharmacoalumite from Cap Garonne -
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