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

Abstract

Wagnerite- Ma 5 bc , 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- Ma 5 bc + 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 P 2 O 5 41.39, SiO 2 0.06, TiO 2 0.88, FeO 4.16, MnO 0.09, MgO 44.54, CaO 0.09, F 6.87, H 2 O (calculated for OH + F = 1) 2.04, O=F −2.89, total 97.22 wt%, corresponding to (Mg 1.88 Fe 0.10 Ti 0.02 )(P 0.99 O 4 )(F 0.61 OH 0.39 ). The space group is Ia . Lattice parameters: a 9.645(2), b 31.659(6), c 11.914(2) A, β 108.26(3)°, V 3455(1) A 3 for Z = 40. The crystal structure has been solved by direct methods and refined to R 1 = 0.0413 for the independent 4521 reflections [I > 2σ(I)] using Mo K α radiation. The primary difference between the wagnerite- Ma 5 bc and wagnerite- Ma 2 bc ( 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- Ma 2 bc , the sequence is ordered 121212… and in wagnerite- Ma 5 bc , 12112... Magniotriplite and the wagnerite polytypes do not overlap in composition: minerals richer in Fe and Mn (average ionic radius ≥ 0.76 A) crystallize as the disordered minerals in the triplite group, whereas highly magnesian minerals (average ionic radius ≤ 0.73 A 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- Ma 2 bc 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 M 2+ 2 PO 4 F compounds originally crystallize in the disordered state, the Mabc polytype, and only magnesian varieties subsequently order on cooling, i.e ., Mn 2+ and Fe 2+ inhibit ordering.