Synthesis and characterisation of ferri-ferroholmquistite, oLi2(Fe2+3Fe3+2)Si8O22(OH)2

This work reports the synthesis and crystal-chemistry of ferri-clinoferroholmquistite in the system Li2O - FeO - Fe2O3 - SiO2 - H2O. Ferri-clinoferroholmquistite has a very restricted temperature stability. An almost monophase amphibole product is obtained at 500°C for a wide pressure range (1-7 kbar), whereas a Li-bearing pyroxene becomes predominant for T ≥ 600°C. The X-ray powder pattern of ferri-clinoferroholmquistite was indexed in the C2/m space group. Refined cell dimensions are: a = 9.489 (2), b = 18.036 (7), c = 5.313 (3) Å; β = 101.59 (3)°. Infrared spectra in the OH-stretching region show the presence of a single and very sharp band at 3614 cm-1, which is assigned to the Fe2+Fe2+Fe2+ trimer at the OH-coordinated M1M1M3 octahedra, and a small satellite band at 3644 cm-1 which was assigned to Li at M3. Mössbauer spectra can be fitted to three doublets assigned to Fe3+ at M2, Fe2+ at M1 and Fe2+ at M3, respectively, with a calculated Fe3+/Fe2+ ratio of 2/3, suggesting a composition close to the nominal one. The spectroscopic data show that synthetic ferri-clinoferroholmquistite has a strongly ordered cation distribution, in agreement with single-crystal refinements done by Caballero et al. (1998) and Oberti et al. (2000) on natural samples with related Li-rich compositions