Crystal structure and chemistry of ferroan phlogopites from the Albano maar lake (Colli Albani volcano, Central Italy)

This work is aimed at the crystal chemical and structural characterization of micas from Colli Albani volcano. In particular all the samples considered in this study, with the exception of sample CC, are from the ending volcanic sequences, usually referred as Albano maar lake. Micas from the first three phreatomagmatic units are homogeneous in composition or show well defined chemical trends. From the forth phreatomagmatic unit upward, chemical composition is more variable, especially in the ejecta. The Fe/(Fe+Mg) ratio varies from 0.06 to 0.55, thus the mica chemical composition ranges from phlogopite to magnesian annite through ferroan phlogopite. Heterovalent octahedral substitutions are related to Ti, Al and very limited amount of Cr. In the interlayer Ba for K substitutions occur up to 0.06 apfu. Crystal structure refinement were carried out in the space group C2/m and C2/c for 1M and 2M1 polytype, respectively (agreement factor, Robs: 0.021 ≤ Robs ≤ 0.035). The crystal structure of micas studied appears to be mostly affected by annitic substitution. An increase in the annitic component of the mica is reflected by a decrease in the distortion affecting both individual tetrahedra (tetrahedral angle variance, TAV: 0.49 ≤ TAV ≤ 1.13°2) and the tetrahedral ring (6.59 ≤ α ≤ 10.34°). Interlayer topology was found to be affected as well. A very well established correlation can be defined between Fe and Ti content and thus an effect of Ti content over the interlayer topology may be introduced also. The deprotonation of the anionic site was found in literature to be a powerful petrogenetic indicator. A well established correlation was found between heterovalent octahedral substitution and anionic site deprotonation. On the contrary heterovalent interlayer substitutions are accompanied by F- for (OH)- substitution.