In order to understand the dependence of Li incorporation and partitioning in the amphibole structure as a function of the intensive parameters (P, T, X, fO2) of crystallisation, a systematic experimental work has been undertaken which combines synthesis of several Li-bearing end-members and characterisation of the products by SEM-EDAX, X-ray diffraction (powder and single-crystal), FTIR and micro-Raman spectroscopy. The present work reports on leakeite, ideally Na Na2 (Mg2Fe3+2Li) Si8 O22(OH)2. Leakeite is Li-bearing amphibole first reported by Hawthorne et al. (1992) in the manganiferous metasediments of the Kajlidongri mine, Madhya Pradesh, India; its composition can be related to that of ferri-eckermannite via the Fe3+ + M3Li ? Mg + M3Mg substitution and to that of magnesio-riebeckite via the ANa + M3Li ? A? + M3Mg substitution. Questa seconda non la sottoscriverei, non c'è bilanciamento di cariche diretto. nota: ci sono altre occurrences Hydrothermal syntheses were done in the T range 500-800 °C and in the P range 0.35-0.90 GPa, with fO2 condition varying from NNO-0.5 to NNO+3; the redox state was constrained either by solid-state or Ar-H2 gas mixtures. The run-products are generally a mixture of leakeite, ferrispodumene and quartz, with minor glass. The relative amount of amphibole and clinopyroxene mainly depends on fO2 conditions, the amphibole being preferred at lower fO2 values. A run at 600 °C, NNO and 0.35 GPa (sample 242) yielded amphibole single-crystals with size suitable for X-ray data collection with a Bruker AXS D8 diffractometer equipped with a SMART Apex CCD detector. Structure refinement and EMP analysis converged to the following crystal-chemical formula: Na1.00 B(Na1.76Li0.24) C(Mg2.40Fe2+0.40Fe3+1.60Li0.60) Si8 O22 (OH)2, where the M2 site is occupied by Fe3+ and Mg, the M1 site by Mg and Fe2+, and the M3 site by Li, Fe2+ and Mg. This ordering pattern is analogous to that observed in natural Li-rich amphiboles (Oberti et al., 2003). Single-crystal micro-Raman and OH-stretching powder FTIR spectra were collected for all samples; both techniques gave very similar results, and show significant variations of the band intensities suggesting that octahedral compositions depend on the run-conditions. The recorded spectra systematically show a broad band at high frequency, typical of amphiboles with fully occupied A-sites (Della Ventura et al., 1999). Three components can be resolved, which are centred at 3748, 3726 and 3703 cm-1, respectively. The two higher-frequency bands can be attributed to the ANa-OH-MgMgLi, and ANa-OH-MgMgMg local configurations, respectively, whereas the lower-frequency band at 3703 cm-1 is related to the incorporation of Fe2+ into the M1 and M3 sites. Della Ventura, G., Hawthorne, F.C., Robert, J.-L., Delbove, F., Welch, M.D., and Raudsepp, M. Eur. J. Miner., 11, 79-94 (1999). Oberti, R., Cámara, F., Ottolini, L., and Caballero, J.-M. Eur. J. Miner., 15, 309-319 (2003). Hawthorne, F.C., Oberti, R., Ungaretti, L., and Grice, J.D. Am. Miner., 77, 1112-1115 (1992).
Synthesis and crystal-chemistry of leakeite, Na Na2 (Mg2Fe3+2Li) Si8 O22(OH)2.
R Oberti
2003
Abstract
In order to understand the dependence of Li incorporation and partitioning in the amphibole structure as a function of the intensive parameters (P, T, X, fO2) of crystallisation, a systematic experimental work has been undertaken which combines synthesis of several Li-bearing end-members and characterisation of the products by SEM-EDAX, X-ray diffraction (powder and single-crystal), FTIR and micro-Raman spectroscopy. The present work reports on leakeite, ideally Na Na2 (Mg2Fe3+2Li) Si8 O22(OH)2. Leakeite is Li-bearing amphibole first reported by Hawthorne et al. (1992) in the manganiferous metasediments of the Kajlidongri mine, Madhya Pradesh, India; its composition can be related to that of ferri-eckermannite via the Fe3+ + M3Li ? Mg + M3Mg substitution and to that of magnesio-riebeckite via the ANa + M3Li ? A? + M3Mg substitution. Questa seconda non la sottoscriverei, non c'è bilanciamento di cariche diretto. nota: ci sono altre occurrences Hydrothermal syntheses were done in the T range 500-800 °C and in the P range 0.35-0.90 GPa, with fO2 condition varying from NNO-0.5 to NNO+3; the redox state was constrained either by solid-state or Ar-H2 gas mixtures. The run-products are generally a mixture of leakeite, ferrispodumene and quartz, with minor glass. The relative amount of amphibole and clinopyroxene mainly depends on fO2 conditions, the amphibole being preferred at lower fO2 values. A run at 600 °C, NNO and 0.35 GPa (sample 242) yielded amphibole single-crystals with size suitable for X-ray data collection with a Bruker AXS D8 diffractometer equipped with a SMART Apex CCD detector. Structure refinement and EMP analysis converged to the following crystal-chemical formula: Na1.00 B(Na1.76Li0.24) C(Mg2.40Fe2+0.40Fe3+1.60Li0.60) Si8 O22 (OH)2, where the M2 site is occupied by Fe3+ and Mg, the M1 site by Mg and Fe2+, and the M3 site by Li, Fe2+ and Mg. This ordering pattern is analogous to that observed in natural Li-rich amphiboles (Oberti et al., 2003). Single-crystal micro-Raman and OH-stretching powder FTIR spectra were collected for all samples; both techniques gave very similar results, and show significant variations of the band intensities suggesting that octahedral compositions depend on the run-conditions. The recorded spectra systematically show a broad band at high frequency, typical of amphiboles with fully occupied A-sites (Della Ventura et al., 1999). Three components can be resolved, which are centred at 3748, 3726 and 3703 cm-1, respectively. The two higher-frequency bands can be attributed to the ANa-OH-MgMgLi, and ANa-OH-MgMgMg local configurations, respectively, whereas the lower-frequency band at 3703 cm-1 is related to the incorporation of Fe2+ into the M1 and M3 sites. Della Ventura, G., Hawthorne, F.C., Robert, J.-L., Delbove, F., Welch, M.D., and Raudsepp, M. Eur. J. Miner., 11, 79-94 (1999). Oberti, R., Cámara, F., Ottolini, L., and Caballero, J.-M. Eur. J. Miner., 15, 309-319 (2003). Hawthorne, F.C., Oberti, R., Ungaretti, L., and Grice, J.D. Am. Miner., 77, 1112-1115 (1992).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
