WAGNERITE-Ma5bc, A NEW POLYTYPE OF Mg2(PO4)(F,OH)
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...
| Main Authors: | , , , , , |
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.538.3384 http://rruff.info/doclib/cm/vol41/CM41_393.pdf |
| Summary: | 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 (Mg1.88Fe0.10Ti0.02)(P0.99O4)(F0.61OH0.39). The space group is Ia. Lattice parameters: a 9.645(2), b 31.659(6), c 11.914(2) Å, 108.26(3)°, V 3455(1) Å3 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 disor-dered, 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 |
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