A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200–1250 °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 + VIAl + VI□ = IVAl + VIMg) and a chemical formula: 1.01(Mg2.45Al0.19□0.35)Σ=3(Si3.52Al0.48) Σ=4O10[(OH,O)1.66Cl0.34)] Σ=2. 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) Å, β =100.06(4)°, V = 489.1(4) Å3. 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 Å from O4 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 α is 0°. 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 α angle and expands its stability field at high pressure.

High aK2O 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.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.