Synthetic hypersilicic Cl-bearing mica in the phlogopite-celadonite join: A multimethodical characterization of the missing link between di- and tri-octahedral micas at high pressures

A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250 degrees C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich aluminosilicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (Al + Mg), suggesting an Al-celadonitic substitution (Si + Al-VI + (VI)square = Al-IV + Mg-VI) and a chemical formula: K-1.01(Mg2.45Al0.19 square(0.35))(Sigma=3)(Si3.52Al0.48)(Sigma=4)O-10[(OH, O)(1.66)Cl-0.34)]. The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 cm(-1) in the Raman spectra. Single-crystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1M polytype): a = 5.299(4), b = 9.167(3), c = 10.226(3) angstrom, beta = 100.06(4)degrees, V=489.1(4) angstrom(3). The structure refinement shows the presence of vacancies on the octahedral sites (15% for M1 and 6.5% for M2). Chlorine occupies a position about 0.5 angstrom from 04 with partial Occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six-tetrahedra ring is hexagonal and the tetrahedral rotation angle a is 0 degrees. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the alpha angle and expands its stability field at high pressure. High a(K2O) conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Al-celadonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds.