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 Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (4) Citing Articles via Google Scholar Google Scholar Articles by Mancini, F. Articles by Cahill, C. Search for Related Content GeoRef GeoRef Citation

The crystal structure and cation ordering of Phase-X-(K_1–x–n)₂(Mg_1–n[Al,Cr]_n)₂Si₂O₇H_2x: A potential K- and H-bearing phase in the mantle^*

Franco Mancini^1,, George E. Harlow¹ and Christopher Cahill²

¹ Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, U.S.A.
² Department of Chemistry, George Washington University, 725 21st Street, N.W. Washington, D.C. 20052, U.S.A.

Correspondence: E-mail: mancini{at}amnh.org

Phase-X, a potassium di-magnesium acid disilicate, is a high-pressure synthetic compound—a potential K-bearing silicate in the mantle—with space group P6₃cm (no. 185), a = b = 5.028(2) Å, c = 13.216(3) Å, V = 289.34 ų, Z = 2. The structure has been determined with 1521 CCD measured intensities and refined by the least-square method to R = 0.0187. The structure is built up of octahedral MgO sheets and layers containing disilicate groups, Si₂O₇, (with distinct Si1 and Si2 tetrahedra linked by the apical O2 atom) alternating along the c axis. The octahedral sheet is based on a hexagonal closest-packed array of two layers of non-equivalent O atoms, O1 and O3; two-thirds of all edge-sharing M octahedra are filled. Within the framework of the Si₂O₇ groups are channel structures parallel to [100], [010], and [110] that contain K atoms disordered in the middle of a large trigonal cavity (the A site). The FTIR spectrum in the OH stretching region shows a sharp peak at 3602 cm^–1 due to OH^– ordered in one anion site; the position of hydrogen, which operates in a charge-balancing substitution for the partial occupancy of the A site (K_1–xx)^A H_x1–x)^H, is undetermined. Densification in phase-X is affected by the greater compression of the empty octahedra in the octahedral layer and by constraining the trigonal A cavity containing the K atom to the size of the Si₂O₇ disilicate group. This dense packing contributes to the relatively high zero-pressure calculated density of 3.38 g/cm³.

This article has been cited by other articles:

				L. Bindi, A. Bobrov, and Y. A. Litvin Incorporation of Fe3+ in phase-X, A2 xM2Si2O7Hx, a potential high-pressure K-rich hydrous silicate in the mantle Mineralogical Magazine, June 1, 2007; 71(3): 265 - 272. [Abstract] [Full Text] [PDF]

				D. J. Frost The Stability of Hydrous Mantle Phases Reviews in Mineralogy and Geochemistry, January 1, 2006; 62(1): 243 - 271. [Full Text] [PDF]

				J. Konzett and S. L. Japel High P-T phase relations and stability of a (21)-hydrous clinopyribole in the system K2O-Na2O-CaO-MgO-Al2O3-SiO2-H2O: An experimental study to 18 GPa American Mineralogist, July 1, 2003; 88(7): 1073 - 1083. [Abstract] [Full Text] [PDF]