Fischer, Reinhard X., Tikhonova, Valeria, Birkenstock, Johannes, Fischer, Lennart A., Herrmann, Klaus, Mengel, Kurt and Schneider, Hartmut (2015). A new mineral from the Bellerberg, Eifel, Germany, intermediate between mullite and sillimanite. Am. Miner., 100 (7). S. 1493 - 1502. CHANTILLY: MINERALOGICAL SOC AMER. ISSN 1945-3027

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Abstract

A mineral intermediate between sillimanite and mullite, tentatively designated as sillimullite, was studied by electron microprobe analyses and single-crystal X-ray diffraction methods. The chemical compositions derived from the microprobe results and the crystal-structure refinement are Al7.84Fe0.18 Ti0.03Mg0.03Si3.92O19.96 and Al8.28Fe0.20Si3.52O19.76 (Fe is Fe3+) corresponding to x-values of 0.02 and 0.12, respectively, in the solid-solution series Al8+4xSi4-4xO20-2x assigning Fe3+, Ti, and Mg to the Al site. The composition derived from microprobe analysis is very close to a stoichiometric sillimanite (with Fe3+,Ti, and Mg assigned to Al sites), while the composition derived from diffraction data is midway between sillimanite and Si-rich mullites. The discrepancy is assumed to be caused by the occurrence of amorphous nano-sized SiO2 inclusions in the aluminosilicate phase not affecting the diffraction data but detected in the microprobe analysis. Sillimullite crystallizes in the orthorhombic space group Pnam with a = 7.5127(4), b = 7.6823(4), c = 5.785(3) angstrom, V = 333.88(4) angstrom(3), Z = 1. It has a complete Si/Al ordering at tetrahedral sites like sillimanite but with neighboring double chains of SiO4 and AlO4 tetrahedra being offset by 1/2 unit cell parallel to c relative to each other causing the change of the space-group setting from Pbnm (sillimanite) to Pnam. Difference Fourier calculations and refinements with anisotropic displacement parameters revealed the formation of oxygen vacancies and triclusters as known in the crystal structures of mullite. Final refinements converged at R1 = 5.9% for 1024 unique reflections with F-o > 4 sigma(F-o). Fe was found to reside predominantly in the octahedral site and with minor amounts in one of the T* sites. Mg and Ti were not considered in the refinements. The crystal studied here is considered to represent a new mineral intermediate between sillimanite and mullite, named sillimullite.

Item Type: Journal Article
Creators:
CreatorsEmailORCIDORCID Put Code
Fischer, Reinhard X.UNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Tikhonova, ValeriaUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Birkenstock, JohannesUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Fischer, Lennart A.UNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Herrmann, KlausUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Mengel, KurtUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
Schneider, HartmutUNSPECIFIEDUNSPECIFIEDUNSPECIFIED
URN: urn:nbn:de:hbz:38-399602
DOI: 10.2138/am-2015-4966
Journal or Publication Title: Am. Miner.
Volume: 100
Number: 7
Page Range: S. 1493 - 1502
Date: 2015
Publisher: MINERALOGICAL SOC AMER
Place of Publication: CHANTILLY
ISSN: 1945-3027
Language: English
Faculty: Unspecified
Divisions: Unspecified
Subjects: no entry
Uncontrolled Keywords:
KeywordsLanguage
AL2SIO5 POLYMORPHS; MOSSBAUER-SPECTROSCOPY; RIETVELD REFINEMENT; CRYSTAL-STRUCTURES; LATTICE-DEFECTS; TRANSFORMATION; ANDALUSITE; SILICA; DIFFRACTION; CHEMISTRYMultiple languages
Geochemistry & Geophysics; MineralogyMultiple languages
URI: http://kups.ub.uni-koeln.de/id/eprint/39960

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