THE CRYSTAL-STRUCTURE AND CHEMISTRY OF HIGH-ALUMINUM PHLOGOPITE

Crystal structure refinements were performed on five Al-rich phlogopit e-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 Hi lls petrographic area (north-east Italy) with the aim of characterizin g geometrical variation produced by Al3+ increase. The charge imbalanc e was mostly compensated for by substitutions of highly charged cation s 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 valu es (R) between 0.025 and 0.030. For some additional crystals, only che mistry 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 tetr ahedral and octahedral sheets, produced by the increase in the tetrahe dral edges, is mostly compensated for by the tetrahedral ring angle ro tation alpha (10.2 degrees less than or equal to alpha less than or eq ual to 12.5 degrees), whereas the octahedral sheet features seem affec ted only by local crystal-chemical variations.