Synthesis, crystal structure and crystal-chemistry of ferri-clinoholmquistite, Li2Mg3Fe3+2Si8O22(OH)2

This work reports the synthesis of ferri-clinoholmquistite, nominally hLi2(Mg3Fe3+ 2)Si8O22(OH)2, at varying fO2 conditions. Amphibole compositions were characterized by X-ray (powder and single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Moessbauer and Raman) techniques. Under reducing conditions (NNO+1, whereNNO=Nickel– Nickel oxide buffer), the amphibole yield is very high (>90%), but its composition, and in particular the FeO/ Fe2O3 ratio, departs significantly from the nominal one. Under oxidizing conditions ( NNO+1.5), the amphibole yield is much lower (<60%, with Li-pyroxene abundant), but its composition is close to the ideal stoichiometry. The exchange vector of relevance for the studied system is M2(Mg,Fe2+) M4(Mg,Fe2+) M2Fe3+ –1 M4Li)1, which is still rather unexplored in natural systems. Amphibole crystals of suitable size for structure refinement were obtained only at 800 C, 0.4 GPa and NNO conditions (sample 152), and have C2/m symmetry. The X-ray powder patterns for all other samples were indexed in the same symmetry; the amphibole closest to ideal composition has a = 9.428(1) A˚ , b = 17.878(3) A˚ , c = 5.282(1) A˚ , b = 102.06(2), V = 870.8(3) A˚^3. Moessbauer spectra show that Fe3+ is strongly ordered at M2 in all samples, whereas Fe2+ is disordered over the B and C sites. FTIR analysis shows that the amount of CFe2+ increases for increasingly reducing conditions. FTIR data also provide strong evidence for slight but significant amounts of Li at the A sites.