Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Bindi, L. Articles by Litvin, Y. A. Search for Related Content GeoRef GeoRef Citation

Incorporation of Fe³⁺ in phase-X, A_2–xM₂Si₂O₇H_x, a potential high-pressure K-rich hydrous silicate in the mantle

¹ Museo di Storia Naturale, sezione di Mineralogia, Università degli Studi di Firenze, Via La Pira, 4, I-50121, Firenze, Italy
² Department of Petrology, Geological Faculty, Moscow State University, Leninsky Gory, 119991, Moscow, Russia
³ Institute of Experimental Mineralogy, Moscow district, 142432, Chernogolovka, Russia

^* E-mail: luca.bindi{at}unifi.it

Phase-X, a potential sink for K in the mantle, is a synthetic hydrous K-rich silicate formed by the breakdown of K-amphibole at high pressure. It has the general formula A_2–xM₂Si₂O₇H_x where A = K, Na, Ca, (vacancy), and M = Mg, Al, or Cr. No other isomorphic substitutions, either for the A or the M site, were reported for such a synthetic compound. Here we report the crystal structure and chemical composition of a crystal of phase-X containing large amounts of trivalent Fe. This crystal was synthesized in the model system garnet lherzolite–K₂CO₃ at lower pressure (P = 7 GPa) and higher temperature (T = 1450–1650°C) with respect to the stability range reported in the literature (i.e. P = 9–17 GPa and T = 1150–1400°C). Quantitative analysis led to the following formula: (K_1.307Na_0.015Ca_0.007) _{= 1.329}(Mg_1.504Fe_0.373³⁺Al_0.053Ti_0.004⁴⁺Mn_0.001²⁺) _{= 1.935}Si₂O_7.00H_0.360. The lattice parameters (hexagonal setting) are: a = 5.005(1), c = 13.148(2) Å, V = 285.23(9) ų, and Z = 2. The structure was refined in space group P6₃cm to R = 5.06% using 199 independent reflections and consists of MO₆ octahedra layers stacked along the c axis and linked together by Si₂O₇ groups. The Si₂O₇ groups form pillars in the layer that contains A atoms in the cavities between the pillars. The Raman spectrum in the OH-stretching region indicates the existence of two different OH environments. However, the position of H could not be determined. The substitution of Fe³⁺ for Mg shortens octahedral bond distances. In addition, the entry of Fe³⁺ in M induces geometrical changes to the adjacent A site. The crystal-chemical characteristics are compared with published data on synthetic Fe-free phase-X.

KEYWORDS: synthetic phase-X, crystal structure, chemical composition, Raman spectroscopy