Bulletin Mineralogie Petrologie

Števko Martin, Tuček Peter, Sejkora Jiří, Malíková Radana

Ročník 24, číslo 2 (2016), strana 183-193

supergene minerals, azurite, baryte, cerussite, malachite, rosasite, smithsonite, wulfenite, chemical composition, Nová Anton vein, Hodruša-Hámre, Slovak Republic, powder X-ray diffraction data

An unusually rich occurrence of supergene minerals, represented by azurite, baryte, cerussite, Zn-rich malachite, rosasite, smithsonite and wulfenite was recently identified at the subsurface parts of the Nová Anton epithermal ore vein near Hodruša-Hámre, Štiavnické vrchy Mts., Slovak Republic. Azurite is rare and it forms bright blue crystalline crusts, which consists of thin tabular crystals up to 2 mm. It is associated together with malachite, cerussite and smithsonite. Baryte occurs only infrequently as white, thin tabular crystals up to 5 mm growing on smithsonite. Cerussite is relatively abundant supergene mineral, especially in the proximity of relicts of primary galena. It forms clear to white, well-developed, acicular, prismatic or tabular crystals up to 1.2 cm or massive aggregates and rims up to 3 cm, which are often associated together with malachite and rosasite. Zn-rich malachite is very common supergene mineral at the studied locality and it occurs as pale green to greenish-blue hemispherical or radial aggregates up to 1 cm or crystalline crusts and coatings, which cover areas up to 100 cm². It is often associated with cerusite or smithsonite with rosasite, rarely also together with wulfenite or azurite. The unit-cell parameters of Zn-rich malachite refined from the powder X-ray diffraction data (for the monoclinic space group P2₁/a) are: a = 9.459(5) Å, b = 11.979(4) Å, c = 3.219(1) Å, β = 97.84(2)° with V = 361.3(3) ų. It contain up to 0.18 apfu of Zn. Rosasite is relatively common species. It forms bluish-green to pale blue, hemispherical aggregates up to 7 mm with radial internal structure, which are often grouped to the botryoidal crusts covering areas up to 50 cm². Rosasite is often associated with smithsonite and Zn-rich malachite. Its refined unit-cell parameters (for the monoclinic space group P2₁/a) are: a = 12.890(8) Å, b = 9.342(7) Å, c = 3.164(2) Å, β = 110.26(2)° with V = 357.4(4) ų. Two types of rosasite can by distinguished by chemical composition. Predominant is Zn-rich phase with the Cu/Zn molar ratio in the range of 1.24 - 1.66 (1.11 - 1.25 apfu Cu, 0.75 - 0.89 apfu Zn). Cu-rich rosasite is infrequent and its Cu/Zn molar ratio vary from 3.32 to 3.61 (1.54 - 1.57 apfu Cu, 0.43 - 0.46 apfu Zn). Smithsonite is very common mineral, which is associated mainly with Zn-rich malachite and rosasite. It occurs as white, pale green to pale blue botryoidal crusts, which often consists of rounded rhombohedral or scalenohedral crystals up to 3 mm in size. Except of major Zn, elevated content of Cu (up to 0.08 apfu), and minor contents of Ca (up to 0.02 apfu) and K, Pb, Mg (all up to 0.01 apfu) were detected in smithsonite. Wulfenite is relatively abundant species and it forms orange to yellow, dipyramidal, often hemihedral or tabular crystals up to 1 cm. It was identified by PXRD and its refined unit-cell parameters (for the tetragonal space group I4₁/a) are: a = 5.438(5) Å, c = 12.1160(7) Å and V = 358.3(3) ų. Its chemical composition is close to the theoretical end member, PbMoO₄. Well-developed supergene zones in-situ, especially with the abudance of supergene Cu and Zn carbonates are not typical for the epithermal ore veins in the Banská Štiavnica ore district. The origin of relatively well developed and rich supergene zone in-situ at the Nová Anton vein is directly linked with the fact, that upper parts of this vein are hosted in limestone.

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