The crystal structure and chemistry of high-aluminium phlogopite

Abstract

Crystal structure refinements were performed on five Al-rich phlogopite-lM crystals (1.50 ~< AI 3+ ~< 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 A13+ increase. The charge imbalance was mostly compensated for by substitutions of highly charged cations in the octahedral sheet (AI 3+ and/or Fe 3+ for Mg2+). The refinements were carried out in the mean space group C21m 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 tetrahe~a 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 cx (10.2 ~ ~< ~< 12.5~ whereas the octahedral sheet features seem affected only by local crystal-chemical variations.

Crystal chemistry of phlogopite from Vulture-S. Michele Subsynthem volcanic rocks (Mt. Vulture, Italy) and volcanological implications

“…This feature seems to reflect the grain-to-grain variability of chemical composition already highlighted above. However, the c parameters observed for the micas of this study are small when compared with those of close-to-end-member annite [10.3235(4) Å, Redhammer and Roth (2002)] and close-to-end-member phlogopite [10.291(4) Å, Alietti et al (1995); 10.3100(54) Å, synthetic sample Phl no. 2 (2002)].…”

Refinement of the Structure of Natural Ferriphlogopite

“…For phlogopites and biotites, the tetrahedral basal bond lengths increase as t4]A1 substitutes for [4]Si and for Al-rich phlogopites, the increased substitution produces a more regular tetrahedra (Alietti et al 1995). Fe 3+, a cation with a radius greater than A13+, emphasizes those effects (Table 4).…”

“…Open symbols= Al-rich phlogopites, phlogopites and biotites from literature (Alietti et al 1995;Bigi and Brigatti 1994;Brigatti and Davoli 1990;Hazen and Burnham, 1973;Joswig 1972;Ohta et al 1982;Takeda and Ross 1975).…”

Mineralogical characterization of the blue pigment of Michelangelo's fresco "The Last Judgment"

“…After analysis of the difference plot, six more phases were identifi ed and added to the refi nement: albite, diopside, nepheline, phlogopite, quartz, and sodalite. The atomic coordinates of these minerals were chosen according to Meneghinello et al (1999), Oberti and Caporuscio (1991), Gregorkiewitz (1984), Alietti et al (1995), Kihara (1990), and Hassan and Grundy (1984). At the fi nal stages of the refi nement the occupancy of the cation and anion sites of lazurite also were refi ned.…”