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WAGNERITE-Ma5bc, A NEW POLYTYPE OF Mg₂(PO₄)(F,OH), FROM GRANULITE-FACIES PARAGNEISS, LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA

Liudong Ren^1,, Edward S. Grew^2,, Ming Xiong³ and Zhesheng Ma³

¹ Institute of Geology, Chinese Academy of Geological Sciences, 100037, Beijing, People’s Republic of China
² Department of Geological Sciences, University of Maine, 5790 Edward T. Bryand Research Center, Orono, Maine 04469-5790, U.S.A.
³ China University of Geosciences, 100083, Beijing, People’s Republic of China

E-mail address: ldren{at}cags.net.cn, esgrew{at}maine.edu

Wagnerite-Ma5bc, a new polytype, occurs in paragneiss associated with banded cordierite–prismatine gneiss in the Larsemann Hills, East Antarctica. It forms anhedral to euhedral grains mostly 0.5–2 mm across, some with a tabular habit. Textures are consistent with a primary assemblage wagnerite-Ma5bc + plagioclase + apatite + magnetite + ilmenite–hematite that crystallized under granulite-facies conditions (750 – ~860°C, 6–7 kbar). Also present are biotite, quartz, K-feldspar, minerals of the monazite and xenotime groups, corundum, hercynite, and sulfide. Electron-microprobe analyses give P₂O₅ 41.39, SiO₂ 0.06, TiO₂ 0.88, FeO 4.16, MnO 0.09, MgO 44.54, CaO 0.09, F 6.87, H₂O (calculated for OH + F = 1) 2.04, O=F –2.89, total 97.22 wt%, corresponding to (Mg_1.88Fe_0.10Ti_0.02)(P_0.99O₄)(F_0.61OH_0.39). The space group is Ia. Lattice parameters: a 9.645(2), b 31.659(6), c 11.914(2) Å, ß 108.26(3)°, V 3455(1) ų for Z = 40. The crystal structure has been solved by direct methods and refined to R₁ = 0.0413 for the independent 4521 reflections [I > 2(I)] using MoK radiation. The primary difference between the wagnerite-Ma5bc and wagnerite-Ma2bc (e.g., type locality) is ordering of the (F,OH) positions. F can occupy one of two positions, resulting in two distinct configurations along the a direction. In magniotriplite, the sequence of configurations in the b direction is disordered, whereas in wagnerite-Ma2bc, the sequence is ordered 121212... and in wagnerite-Ma5bc, 12112... Magniotriplite and the wagnerite polytypes do not overlap in composition: minerals richer in Fe and Mn (average ionic radius 0.76 Å) crystallize as the disordered minerals in the triplite group, whereas highly magnesian minerals (average ionic radius 0.73 Å or 86% of the Mg end member) crystallize as the ordered polytypes of wagnerite. Magniotriplite formed at moderate temperatures (e.g., amphibolite-facies conditions), whereas wagnerite-Ma2bc is found in rocks formed under a wide range of P–T conditions. Compositional or kinetic factors (or both), rather than P–T, could play the leading role in determining the extent of F order; possibly many M²⁺₂PO₄F compounds originally crystallize in the disordered state, the Mabc polytype, and only magnesian varieties subsequently order on cooling, i.e., Mn²⁺ and Fe²⁺ inhibit ordering.

Keywords: wagnerite, magniotriplite, crystal structure, polytype, granulite facies, Larsemann Hills, Antarctica.

This article has been cited by other articles:

				E. S. Grew, T. Armbruster, O. Medenbach, M. G. Yates, and C. J. Carson TASSIEITE, (Na,{square})Ca2(Mg,Fe2+,Fe3+)2(Fe3+,Mg)2(Fe2+,Mg)2(PO4)6{middle dot}2H2O, A NEW HYDROTHERMAL WICKSITE-GROUP MINERAL IN FLUORAPATITE NODULES FROM GRANULITE-FACIES PARAGNEISS IN THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA Can Mineral, April 1, 2007; 45(2): 293 - 305. [Abstract] [Full Text] [PDF]