Chrystallochemistry and genesis of vermiculites in Bulgaria

The layer silicates in the vermiculite deposits and ore-manifestations in Bulgaria were studied on the basis of X-ray diffractometric studies. The vermiculite deposits and their embedding rocks contain vermiculite, biotite, phlogopite, randomly interstratified biotite-vermiculite and hydrobiotites (ordered 1:1 interstratified biotite-vermiculite). Chlorite and talc were established in single samples. The following crystallochemical features of the phyllosilicates registered were established: 1) all intermediate and end members of the series biotite — phlogopite from polytype modifications 1M are present; 2) Mg-vermiculite, some of which contain small amounts of Ca-exchange cations, were registered predominantly; 3) Mg-rich trioctahedral chlorites IIb were established in single samples; 4) ordered and randomly interstratified biotite-vermiculite with different amounts of vermiculite layers and Ca-exchange cations were established. A vertical zonation in distribution of the phyllosilicates in the vermiculite deposits was established as follows: biotite-phlogopites, interstratified biotite-vermiculites and vermiculites. The availability of the series biotite-phlogopite → interstratified biotite-vermiculite → vermiculite is considered as an indication for the vermiculite origin through hydratation of the initial trioctahedral micas with intermediate members, represented by interstatified biotite-vermiculites with different proportions of biotite/vermiculite.