Acta Musei Moraviae

Mineralogy of massive serpentine and clinochlore from the dolomitic marble of Moldanubicum in West Moravia (Bohemian Massif)

Massive aggregates of yellow-green, grey-green and brown Mg-rich phyllosilicates occur in dolomite marbles enclosed in Moldanubian gneisses and especially migmatites (Bohemian Massif). Serpentine subgroup minerals and Mg-chlorites were distinguished on the basis of X-ray and chemical studies. The serpentine is represented by Al- and Fe-poor lizardite (~0.01 apfu Al; 0.01–0.05 apfu Fe). The Mg-chlorite belongs to the clinochlore (so-called pseudophite), which, in addition to the significant content of Al (1.54–2.32 apfu), has variable content of Fe (0.03–1.07 apfu) and relatively low contents of Mn and Ca. Trace contents of Zn (~0.02 apfu) in both lizardite and clinochlore from the localities where the marbles have elevated Zn > 500 ppm are notable. Only microscopic phlogopite, fluorapatite, muscovite, titanite and tremolite were identified in the studied massive Mg-phyllosilicates. Serpentine (lizardite) was formed as part of metasomatic reaction veins (calcite-forsterite-diopside skarn), where it forms a separate monomineral zone. Clinochlore was formed by the complete replacement of plagioclase and K-feldspar along the contact of granitoid veins (metatects) intruding to the dolomite marbles, and the veins simultaneously underwent desilification. In both cases, they are products of reactions of dolomite marbles with external fluids and/or melts rich in SiO₂ ± Al₂O₃ at high H₂O activities (X_CO2 < 0.10–0.05) and at T < 300°C.