Mixed-layer crystal structures of the hydrobiotite type in tuffs of the Doupovské hory Mts. region

K. MELKA¹, M. ADAMOVÁ², I. HALADOVÁ²

¹ Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Praha 6, Czech Republic

² Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic

Key words: phlogopite-bearing tuff, Oleška near Doupov, regular mixed layer structure - hydrobiotite, X-ray powder identification, phlogopite-hydrobiotite-smectite transition

Abstract

Mineralogical investigation of mica flakes occurring in tuffs of the Doupovské hory Mts. was carried out as a part of a regional study on volcaniclastic rocks. The volcanic area of the Doupovské hory Mts. belongs to the extensive Ohøe Rift originated in Early Cenozoic time.

The phlogopite-bearing tuff, green-grey in colour , was collected from an outcrop at Oleška near Doupov. Based on geochemical criteria the volcaniclastic rock studied contains some tephra clasts derived from alkali basalt-basanite rocks.

Analyses of separated mica flakes, several millimetres in size, demonstrated a reduced amount of K₂O and enhanced amount of H₂O in contrast to fresh mica. It is consistent with the chemical composition of hydrobiotites. The X-ray diffractograms gave mean basal spacing d₀₀₁ with the value of 24.51 + 0.51 Ã, calculated of twelve 00l basal diffractions. Low angle diffraction confirms the presence of the regular mixed-layer structure, 1 : 1 interstratification of phlogopite and vermiculite layers is supposed. Hydrobiotite as a specific mineral name is reserved for such crystal structures. X-ray powder identification confirmed the existence of the mineral of hydrobiotite type as well as the presence of phlogopite , smectite, and kaolinite. Compositional study of separated mica flakes exhibited continuous phlogopite- hydrobiotite-smectite transitions in them.