THE CRYSTAL-STRUCTURE AND CHEMISTRY OF HIGH-ALUMINUM PHLOGOPITE

Crystal structure refinements were performed on five Al-rich phlogopite-1M crystals (1.50 less than or equal to Al3+ less than or equal to 1.97 atoms per formula unit) from skarns of the Predazzo and Monzoni Hills petrographic area (north-east Italy) with the aim of characterizing geometrical variation produced by Al3+ increase. The charge imbalance was mostly compensated for by substitutions of highly charged cations in the octahedral sheet (Al3+ and/or Fe3+ for Mg2+). The refinements were carried out in the mean space group C2/m and gave agreement values (R) between 0.025 and 0.030. For some additional crystals, only chemistry and/or unit cell parameters were determined. In all samples the tetrahedra are more regular and larger than those previously reported in the literature for phlogopite crystals and the misfit between tetrahedral and octahedral sheets, produced by the increase in the tetrahedral edges, is mostly compensated for by the tetrahedral ring angle rotation alpha (10.2 degrees less than or equal to alpha less than or equal to 12.5 degrees), whereas the octahedral sheet features seem affected only by local crystal-chemical variations.