On the reported presence of potassium in clinopyroxene from potassium-rich lavas: a transmission electron microscope study

Abstract

AbstractTitanian aegirine in a pegmatoid differentiate from leucitite sensu stricto (i.e. without modal plagioclase) was found to contain significant K (up to 0.07K atoms per formula unit; 0.11 wt.% K2O) by electron probe microanalysis. A transmission electron microscope study showed that this pyroxene contained a lamellar amorphous phase and other more irregular, amorphous domains where K and Al are concentrated. The microstructures suggest that the aegirine-amorphous phase system did not achieve textural equilibrium. Also, the chemistry of this phase is variable, particularly in Ca and Mg, suggesting lack of chemical equilibrium within the liquid from which the phase derived. The average composition of the latter approaches that of the coexisting K-richterite and is interpreted as trapped residual liquid, i.e. glass, from pyroxene crystallization which failed to develop a crystalline amphibole. In view of the extensive stability of richterite coexisting with pyroxene to high pressure, the reported occurrence of ‘K-bearing pyroxene’ may not be considered as a homogeneous K-bearing phase and a predictable potassium carrier in the petrogenesis of K-rich rocks like leucitites and lamproites.