Sign on

SAO/NASA ADS Physics Abstract Service

· Find Similar Abstracts (with default settings below)
· Also-Read Articles (Reads History)
·
· Translate This Page
Title:
Polytypism in wagnerite, Mg2PO4(F,OH)
Authors:
Chopin, C.; Armbruster, T.; Leyx, C.
Publication:
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #8323
Publication Date:
04/2003
Origin:
EGU
Bibliographic Code:
2003EAEJA.....8323C

Abstract

The Mg, Fe and Mn phosphates with formula Me2+_2PO_4(F,OH) belong to two groups which share the same fundamental monoclinic structure type, but in one of which the b parameter is doubled. Specifically, magniotriplite (Mg), zwieselite (Fe) and triplite (Mn) are F-dominant and have space group I2/a, with Z = 8, b_0 ≈ 6.5 Å, whereas wagnerite (Mg, F dominant), wolfeite (Fe, OH dominant) and triploidite (Mn, OH dominant) have space group P2_1/a, Z = 16 and b = 2 b_0 ≈ 13 Å. In I2/a magniotriplite, eight F atoms are distributed over two eightfold equipoints with 50% occupancy. Periodic ordering of the F atoms into each of these equipoint positions (say A and B), each alternately void and fully occupied along b, leads to a new, double cell with space group P2_1/a and a 2b_0 parameter, i.e. the wagnerite cell, in which the succession of the occupied F positions along b is ABAB (Tadini 1981). Ren et al. (2002) reported from granulite-facies rocks of East Antarctica a wagnerite "polymorph" structurally very close to wagnerite, but with space group Ia, b = 5b_0 ≈ 32 Å and Z = 40. We studied wagnerite crystals from granulite-facies rocks of central Australia (Vry and Cartwright 1994). CCD area-detector imaging revealed either 7b_0 ≈ 45 Å or 9b_0 ≈ 57 Å superstructures (Z = 56 and 72, respectively). The structure of the 9b_0 phase was refined in space group Ia to R = 6% from 11903 unique reflections. We show that wagnerite and the 5b_0, 7b_0 and 9b_0 phases share the same topological arrangement of cations and oxygen atoms, differ only by the periodic faulting of the A-B succession of the F atoms along b, and are all members of a polytypic series based on the magniotriplite cell (b_0). The relevant polytypes and F ordering schemes are wagnerite-a2bc (AB), wagnerite-a5bc (ABAAB), wagnerite-a7bc (ABAABAB) and wagnerite-a9bc (ABAABABAB). Reinvestigation of OH-rich wagnerite from Miregn (Central Alps) and of OH-rich to OH-dominant wagnerite from Dora-Maira (W. Alps) confirmed the original 2b polytype. The question arises whether the longer-period polytypes are not restricted to F-rich members, i.e. whether OH groups stabilize the most ordered configuration (2b) or, depending on the P and T conditions, other, completely different polymorphic structures that only exist for the Me2+_2PO_4(OH) members (cf. holtedahlite-satterlyite, althausite). Ren L., Grew E.S., Xiong M., Ma Z. (2002) GSA meeting, Denver, abstr. No. 234-8. Tadini C. (1981) Bull. Minéral. 104, 677-680. Vry J.K., Cartwright I. (1994) Contrib. Mineral. Petrol. 116, 78-91.
Bibtex entry for this abstract   Preferred format for this abstract (see Preferences)
   

Find Similar Abstracts:

Use: Authors
Title
Abstract Text
Return: Query Results Return    items starting with number
Query Form
Database: Astronomy
Physics
arXiv e-prints