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Synthetic P2₁/m amphiboles in the system Li₂O-Na₂O-MgO-SiO₂-H₂O (LNMSH)

¹ Dipartimento di Scienze della Terra, Università G. D’Annunzio, I-66013 Chieti Scalo, Italy
² Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy
³ Dipartimento di Scienze della Terra, Università di Parma, Parco Area delle Scienze 157/A, I-43100 Parma, Italy

Correspondence: ^* E-mail: g.iezzi{at}unich.it

We describe here the synthesis of amphiboles along the nominal Na(NaMg)Mg₅Si₈O₂₂(OH)₂–Na(LiMg)Mg₅Si₈O₂₂(OH)₂ join, at 800 °C, 0.4 GPa. High amphibole yields (>90%) plus minor quartz and enstatite have been obtained at all compositions; amphibole crystals are acicular and their size rarely exceeds 20–30 x 0.5–3 µm. TEM analysis shows the presence of h+k odd reflections in all samples, indicative of a P-lattice. By similarity with closely related amphiboles from the literature (e.g., Oberti et al. 2000; Cámara et al. 2003) a P2₁/m space group was assigned to the amphiboles synthesized here. Refined cell-parameters from X-ray powder-patterns show a linear decrease as a function of increasing Li at M4, a and ß being the most affected parameters. The four infrared OH-stretching spectra all show two main bands at 3741–3748 and 3712–3716 cm^–1, respectively. They are assigned to two independent O-H groups in the P2₁/m structure, interacting with a strongly delocalized ^ANa. The spectra show in addition two minor absorptions at about 3688 and 3667 cm^–1, respectively; these bands are assigned to vacant A-sites in the structure and indicate slight departure of the nominal composition toward cummingtonite. The present work shows that one apfu of ^BNa can also be completely replaced by one apfu of ^BLi (M⁺), in synthetic Na(M+Mg)Mg₅Si₈O₂₂(OH)₂, and that all compositions have P2₁/m symmetry at ambient conditions.

Key Words: amphibole synthesis • FTIR spectroscopy • TEM • XRPD • amphibole symmetry

This article has been cited by other articles:

				G. D. Ventura, F. Bellatreccia, G. Iezzi, R. Oberti, and F. Camara The P21/m-C2/m phase transition in synthetic amphiboles in the system Li2O-Na2O-MgO-SiO2-H2O: a high-T FTIR study European Journal of Mineralogy, December 1, 2008; 20(6): 1019 - 1026. [Abstract] [Full Text] [PDF]

				R. Oberti, G. D. Ventura, and F. Camara New Amphibole Compositions: Natural and Synthetic Reviews in Mineralogy and Geochemistry, October 1, 2007; 67(1): 89 - 124. [Full Text] [PDF]

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