Acta Musei Moraviae

Occurence and mineralogy of green chalcedony (plasma) in serpentinite residues at Moravia (Moldanubian Zone, Bohemian Massif)

Massive green chalcedony (plasma) represents a specific mineral in the weathering zone at serpentinized peridotites of western Moravia (Moldanubian Zone, Bohemian Massif). Plasma consists mainly of microand non-crystalline silica minerals with different proportions of H₂O (chalcedony >> quartzine, opal-CT, moganite), with finely dispersed green, brown and reddish-brown pigment (chlorite and clay minerals, Fe-oxohydroxides). Relics of accessory minerals from protolite rock (peridotite), mainly amphiboles, Cr-diopside and Cr-spinels are locally also common. A very rare but genetically important are Sporady pyrite, barite and “carbonate-apatite”. Typical microscopic features in most samples of plasma are wormlike SiO₂ “ghost texture” in their marginal zones. The term plasma should be used in the sense of the original definition for fine-grained “siliceous materials” green-colored of finely dispersed phyllosilicates (chlorite, clay minerals), poor in H₂O. From the texture-mineralogical point of view can be divided by two main subtypes. First, widespread subtype, are light green strongly inhomogeneous types relative rich of inclusions (e.g. chlorite aggregates). The second subtype is homogeneous, dark green and often zoned “gem plasma” without significant microscopic inclusions of other minerals. Plasma tends to have zonal structure, with a dark core, greenish light rim and a white narrow margin. Typical nodular green to dark-green plasma occurs with small exceptions only at localities at very limited area in south-western Moravia. Origin of plasma is intimately related to the weathering of serpentinized peridotites, which took place in different geochemical environments and in the different stages. The whole process had to proceed alterations of serpentinite (mainly chloritization), probably in connection with migmatization of surrounding gneiss. The release of SiO₂, which form a gel absorbed clay component weathering occurred during decomposition of serpentinite under tropical conditions during the Cretaceous to Paleogene. Plasma was originally joined to chlorite-clay rich serpentinite residues, which were formed in part inside reducing environment, presumably in interacting with marine Tertiary sediments. Subsequent processes resulted in SiO₂-redistribution and recrystallization silica minerals to quartz and also precipitation of opal in the edge of nodules in later stages.