Crystal chemistry of ferroan phlogopites from the Albano maar lake (Colli Albani Volcano, central Italy)

In the micas present both as phenocrysts in granular xenoliths and as large xenocrysts in the phreatomagmatic deposits characterizing the final volcanic activity of the Colli Albani volcanic complex, the Fe/(Fe + Mg) ratios range from 0.06 to 0.55, i.e. from phlogopite to magnesian annite through ferroan phlogopite. Heterovalent octahedral substitutions involve Ti, Al and also Cr but in very limited amounts. In the interlayer, Ba for K substitution occurs up to 0.06 pfu. Crystal structure refinements were determined in space groups C2/m and C2/c for the 1M and 2M (sub 1) polytypes, respectively (R 0.021-0.035). The crystal structure of these micas is greatly affected by the annitic substitution; an increase in the annitic component is reflected by a decrease in distortion affecting both individual tetrahedra and the entire tetrahedral ring, and also the interlayer topology. A strong correlation exists between Fe and Ti, thus the same correlations found for Fe apply to Ti also. A strong correlation also exists between heterovalent octahedral substitutions and the (OH) (super -) site deprotonation. In contrast, heterovalent interlayer substitutions are accompanied by substitution of F (super -) for (OH) (super -) . Despite their scattered occurrence in pyroclastic rocks, these micas nicely represent the overall crystal chemical trends of micas which recrystallized undisturbed in magma before the onset of explosive conditions.