Sign on

SAO/NASA ADS Physics Abstract Service

· Find Similar Abstracts (with default settings below)
· Electronic On-line Article (HTML)
· References in the Article
· Citations to the Article (7) (Citation History)
· Refereed Citations to the Article
· Reads History
·
· Translate This Page
Title:
Ellenbergerite, a new high-pressure Mg-Al-(Ti,Zr)-silicate with a novel structure based on face-sharing octahedra
Authors:
Chopin, Christian; Klaska, Rolf; Medenbach, Olaf; Dron, Dominique
Affiliation:
AA(Laboratoire de Géologie, Ecole Normale Supérieure, E.R. 224), AB(Institut für Mineralogie, Ruhr-Universität), AC(Institut für Mineralogie, Ruhr-Universität), AD(Laboratoire de Géologie, Ecole Normale Supérieure, E.R. 224)
Publication:
Contributions to Mineralogy and Petrology, Volume 92, Issue 3, pp.316-321 (CoMP Homepage)
Publication Date:
09/1986
Origin:
SPRINGER
Abstract Copyright:
(c) 1986: Springer-Verlag
DOI:
10.1007/BF00572160
Bibliographic Code:
1986CoMP...92..316C

Abstract

Ellenbergerite occurs as purple millimetre-size grains associated with talc, kyanite, clinochlore, rutile, and zircon in composite inclusions within decimetre-large pyrope crystals (90 98 mole percent end-member) in the quartzite layer of the Dora Maira massif, Western Alps, from which coesite has been recently reported (Chopin 1984). It is hexagonal, a=12.255(8), c=4.932(4) Å, Z=1, space group P63. Mohs hardness 6.5; Dmes 3.15, Dcal 3.10; no cleavage. Uniaxial negative and vividly pleochroic, ω colourless, ɛ colourless to deep lilac with colour zoning. The intensely coloured variety has ω 1.6789(5), ɛ 1.670(1); microprobe analysis yields SiO2 39.1, P2O5 0.45, Al2O3 25.1, TiO2 4.0, MgO 22.2, FeO 0.20, sum 99.05 wt.% including H2O 8.0 (coulometrically). The formula calculated on a O28(OH)10 basis (implying 7.5 wt.% H2O) is Mg6.71 Fe0.03 Ti0.61 Al6.00 Si7.92 P0.08 O28(OH)10 The colour zoning is due to nearly complete Ti⇌Zr substitution. In addition ellenbergerite may contain more than 8 wt.% P2O5 with strictly correlated changes of Si, Mg, Al and Ti+Zr contents, over 80% of which represent the SiAl⇌PMg substitution. The structure has been determined from 1049 observed independent reflections and refined to R=0.034, Rw=0.031, including six of ten protons. It consists of single chains of face-sharing octahedra with one third vacancies extending along the six-fold screw axes, and of pairs of fully occupied face-sharing octahedra linked by edge-sharing to form octahedral double chains parallel to the twofold screw axes, all interconnected by SiO4 tetrahedra. It may be compared with the dumortierite polymorph with space group P63mc derived hypothetically by Moore and Araki (1978). The structural formula is (Mg,Ti,Zr,□)2 Mg6(Al,Mg)6 (Si,P)2 Si6 O28(OH)10 Face-sharing octahedra are an unusual feature in silicates which results in a dense structure and reflects, considering the common bulk composition, the uncommon high-pressure formation conditions (about 25 30 kbar, 700 800° C). Ti4+-Fe2+ charge transfer between face-sharing octahedra on the six-fold screw axes most likely accounts for the absorption scheme.
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