Oxide, coronadite group
Specific gravity: 5.285 calculated
Thalliomelane is a relatively new mineral, approved in 2019 and to date (June 2022) reported only from the type
At the type locality, the Zalas quarry, Gmina Krzeszowice, Kraków County, Lesser Poland Voivodeship, Poland, thalliomelane was discovered in relics of supergene mineralisation disseminated in a fault breccia in sandy limestone. It occurs rarely and exclusively in the form of fibrous and highly porous tiny aggregates <50 µm in size that fill small fractures and voids in the sandy limestone host rock.
The formation of thalliomelane was most probably connected to the weathering of a sulphide mineral assemblage under semi-arid to arid conditions. It resulted in the release of thallium and other components of the mineralisation into water under the influence of chlorine-, bromium- and iodine-bearing brines or from sediments. Due to the interaction of these waters, the primary ores altered, mainly into goethite, cuprite, malachite and manganese oxides of the coronadite type, with subordinate copper sulphates, lead arsenates, bismuth oxy-chlorides and traces of iodargyrite.
This assemblage indicates oxidation at a progressively increasing pH (alkalinity) of around 8 to 10, where thalliomelane could have formed from a cryptomelane-type manganese oxide in contact with thallium-bearing aqueous solutions (AM 106.2020-2027).
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