Cryptomelane, K(Mn~(4+)_7Mn~(3+))O_(16), from the Dubravica-Zolnica Fe deposit (North Veporic Unit, Slovak Republic)

摘要

Cryptomelane, K(Mn~(4+)_7Mn~(3+))O_(16), representing a member of the coronadite group and hollandite supergroup, is described from the abandoned Fe deposit Dubravica-Zolnica (near the town of Banska Bystrica, Central Slovakia). Cryptomelane occurs in association with two generations of goethite and limonite. Cryptomelane is located on the cracks and in cavities of Lower Triassic quartzites, arkoses and sandstones of the Luzna Formation (North Veporic Unit). It forms net of irregular veins of the steel grey to blue grey colour and botryoidal aggregates of steel grey colour. The thickness of veins is up to 3 mm. Botryoidal aggregates occur on the area of up to 2 x 3 mm. The lustre is in both cases earthy to dull, or metallic. Cryptomelane was confirmed by Raman spectroscopy and EPMA-WDS analyses. Its empirical formula is (in the mean of 8 point analyses): (K0.64Na0.06Ba0.02Sr0.01Ca0.01Zn0.01 Co0.01)_(Σ0.76)[(Mn~(4+)_(6.97)U~(6+)_(0.02)P_(0.01))_(Σ7.00) (Mn~(3+)_(0.83)Fe~(3+)_(0.12))_(Σ0.95)]O16.00. Cryptomelane has a tunnel structure, in the case of this study these tunnels are occupied by water molecules. It is a weathering product of primary manganese mineral phases (probably rhodonite, pyrolusite or braunite). Formation of cryptomelane indicates slight change in pH and Eh. It precipitates at higher pH values, than associated Fe oxihydroxide minerals. The coexistence of iron and manganese oxidic minerals well corresponds to iron residual deposit.