Natural system of fornacitc -vauquelinitc—embreyite solid solutions and variations in the chemical composition of these minerals from occurrences of the Urals

Khanin D. A., Pekov I. V., Pakunova A. V., Ekimenkova I. A., Yapaskurt V. O.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2015. V. 144. N 4. P. 36-60

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Language: Russian

Abstract

Chemical variations of fornacite, vauquelinite and embreyite from the oxidation zone of six deposits (all at the Urals, Russia) were studied on the base of 426 analyses including 386 new ones. For the first time it was found that these three minerals form a complex, continuous solid-solution system subdivided into two branches: (1) a complete isomorphous series between the isostructural fornacite Pb₂Cu(CrO₄)(AsO₄)(OH) and vauquelinite Pb₂Cu(CrO₄)(PO₄)(OH), with the strongly prevailing substituon scheme As⁵⁺ ↔ P⁵⁺ (correlation coefficient –0.98); (2) an almost complete, continuous series between non-isostructural vauquelinite and embreyite, with the following major varying value: (Pb + Ca):ΣM’(M’ = Cu + Ni + Al + Fe) (correlation coefficient –0.82) or, more simply, the Pb:Cu ratio (correlation coefficient –0.81). The crystal structure of embreyite is still unknown, and this mineral strongly differs from vauquelinite in the powder XRD data. A significant direct substitution Pb ↔ M’ seems very hardly probable, therefore, it is assumed that the embreyite structure contains a pair (or a group) of non-equivalent positions, one of which is favorable for Pb and another for smaller M’ cations, typically with strong predominance of Cu; probably these positions cannot be occupied together. Taking into account new chemical data, we propose the following general formula of embreyite: Pb₂[Pb_xM'²⁺_y□_1-x-y]_Σ1 (CrO₄)(PO₄)(OH_2(x+y)-1,H₂O,□)_Σ1, with Cu and Zn as the major M²⁺ cations and 0.5 ≤ x + y ≤ 1; a content of the above-mentioned hypothetic pair (group) of the M’ positions is given in square brackets. A novel, zinc-enriched (Zn > Cu) variety of embreyite has been discovered at the Sukhovyaz deposit, the South Urals.

Key words: fornacite, vauquelinite, embreyite, natural lead chromate, oxidation zone, Berezovskoe deposit, Urals.