Synthesis, crystal structure and crystal-chemistry of ferri-clinoholmquistites, Li2 [(Mg,Fe)2+2Fe3] Si8 O22(OH)2.

The work reported here is a part of a systematic investigation of natural and synthetic Li-bearing amphiboles, and is focussed on hydrothermal synthesis and characterisation of the nominal end-member ferri-clinoholmquistite, ? Li2 (Mg3Fe3+2) Si8 O22(OH)2, at varying T, P and fO2 conditions. Syntheses were carried-out in internally-heated vessels; at reducing conditions, a Shaw membrane and a Ar-H2 gas mixture was used, for more reduced oxygen fugacity conditions, and b) one pressurised by Ar for lower hydrogen fugacity, in which the redox state is controlled by the EMF measure mean (Pownceby & O´Neill, 1994). A Li-rich amphibole is stable at T between 600 and 800 °C at any pressure within the investigated range (0.1<= PH2O <= 0.55 GPa); at 900 °C, it is completely replaced by an assemblage of Li-pyroxene and clinoenstatite, whereas below 600°C it is replaced by Li-pyroxene + talc + lepidolite + quartz. The end-member ? Li2 (Fe2+3Fe3+2) Si8 O22(OH)2 has been recently investigated in a strictly analogous way by Iezzi et al. (2003); run at T = 500 °C and P in the range 0.1-0.7 GPa yielded monophase amphibole, whereas run at T ? 600°C yielded dominant ferrispodumene (+ quartz). The stability at lower T could not be studied because of the very slow kinetics of the reactions. Anyway, the amphibole obtained at any P value is close to stoichiometry, with only a vary small amount (< 5% ) of M3Li detected by means of a very weak band in the IR spectrum. No reversal experiments were done to check for stability. However, the same products were obtained for both the end-members at the same P, T, and fO2 conditions when starting either from hydroxide-oxides mixture or from gels. Also, the crystallisation conditions inferred by Caballero et al. (1998) for nature-occurring sodic ferriclinoferroholmquistite are coherent with those obtained by the present work. The amphiboles obtained under the different conditions were characterised by a combination of X-ray (powder + single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Mössbauer and Raman) techniques. Under oxidising conditions (>= NNO+1.5), the products are Li-pyroxene (dominant) + amphibole + quartz. However, the amphibole composition is close to the ideal stoichiometry. All the synthetic amphiboles have C2/m symmetry, and the one closest to ideal composition has a = 9.428(1) Å, b = 17.878(3) Å, c = 5.282(1) Å, ? = 102.06(2°), V = 870.8(3) Å3 (from Rietvield refinement). A single-crystal suitable size for structure refinement was obtained at 800 °C, 0.4 GPa and NNO conditions. The resulting crystal-chemical formula based on structural, chemical and spectroscopic analysis is: ? (Li1.04Fe2+0.34Mg0.62) (Mg2.40Fe2+0.60) (Fe3+1.04Mg0.96)Si8O22(OH)2. With this composition as reference, some conclusions could be drawn also for the other samples whose crystal size allowed only spectroscopic analysis. Fe3+ is generally ordered at the M2 site, whereas the Fe2+ content at both the B- and C- group sites increases with T and decreases with the redox state. Stoichiometric ferri-clinoholmquistite is obtained only under very oxidising conditions. Further syntheses were designed to investigate the relative thermal stability between ferri-clinoferroholmquistite and ferriclinoholmquistite. Seven compositions at the extremes and within the solid solution (Fe2+Mg-1) were synthesised at 500 °C and 600 °C, respectively NNO??. At 500 °C, nominal ferri-clinoferroholmquistite incorporates up to 1.5 Mg apfu; at 600 °C nominal ferri-clinoholmquistite incorporates up to 1.5 Fe2+apfu. The amphibole composition of all these solid-solution terms could only be inferred by IR spectroscopy, due to the extremely small size of the synthetic crystals, which prevented EMP analysis. The exchange vector of relevance for this system is therefore M2(Mg, Fe2+) M4(Mg,Fe2+) M2Fe3+-1 M4Li-1 which, together with the analogous M2(Mg, Fe2+) M4(Ca) M2Fe3+-1 M4Li-1 exchange, is still rather unexplored in natural systems. REFERENCES Caballero, J.-M., Monge, A., La Iglesia, A., Tornos, F. (1998): Ferri-clinoholmquistite, Li2(Fe2+,Mg)Fe3+2Si8O22(OH)2, a new BLi clinoamphibole from the Pedriza Massif, Sierra de Guadarrama, Spanish Central System. Am. Mineral., 83, 167-171 Oberti, R., Cámara, F., Ottolini, L. and Caballero, J.-M. (2003) Lithium in amphiboles: detection, quantification, and incorporation mechanisms in the compositional space bridging sodic and BLi-amphiboles. European Journal of Mineralogy, in press. Pownceby, M.I. et O'Neill, H.St.C. (1994). Thermodynamic data from redox reactions at high temperatures. III. Activity-composition relations in NI-Pd alloys from EMF measureaments at 850-1250 K, and calibration of the NiO+Ni-Pd assemblages as redox sensor. Contributions to Mineralogy and Petrology, 116, 327-339.