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Formula: Sc(AsO4).2H2O
Hydrated arsenate of scandium,
metavariscite group
Crystal system: Monoclinic
Specific gravity: 2.82 calculated
Hardness: 3½ to 4
Streak: White
Colour: Colourless (with faint to distinct violet tints)
Luminescence: No fluorescence under UV
Environments
Metamorphic environments
Hydrothermal environments
Bonacinaite is a relatively new mineral, approved in 2018 and to date (May 2025) reported only from the type
locality.
Localities
At the type locality, the Varenche Mine, Saint-Barthélemy, Nus, Aosta Valley, Italy, bonacinaite occurs very
rarely in the dumps of the abandoned old manganese mine. The exploited ore
formed a bench about 1 m thick and consisted of braunite, spotted by
spessartine stains and thin veins. The ore formed in a submarine
environment by exhalative hydrothermal activity related to ophiolitic
volcanism and subsequently was recrystallised and deformed.
Bonacinaite forms very small, up to 0.25 mm in size, pseudohexagonal, thick tabular, transparent, crystals with
a violet tint in a small void with associated quartz, granular
braunite, undefined manganese
oxides, arseniopleite,
manganberzeliite and
thortveitite. The violet tints in bonacinaite are
attributed to trace contents of Mn3+. Only about a dozen small specimens, each with a very small number of
crystals, are known.
The presence of scandium in the deposit is reflected by the localised
occurrence of a fair amount of blue thortveitite, closely associated
with arseniopleite, in a small, humid section of the upper tunnel. In
contrast, thortveitite is rare in the dump. Evidently, the small size
of the thortveitite-containing area in the tunnel is the reason that
thortveitite and
arseniopleite are only rarely found in the dump, as is the even rarer
bonacinaite.
Tentatively, it is inferred that bonacinaite is a low-temperature hydrothermal phase whose primary sources of
scandium and arsenic may have been
thortveitite and
arseniopleite and/or
manganberzeliite, respectively, which are typically associated
with it
(EJM 36.5.863-872).
Bonacinaite from the Varenche Mine -
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