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A new mineral from the Bellerberg, Eifel, Germany, intermediate between mullite and sillimanite

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Published/Copyright: June 30, 2015
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American Mineralogist
From the journal American Mineralogist Volume 100 Issue 7

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) Å, V = 333.88(4) Å3, 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 ½ 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 Fo > 4s(Fo). 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.”

Keywords: Sillimullite; new mineral; crystal structure; electron microprobe; sillimanite; mullite
Received: 2014-3-9
Accepted: 2015-1-20
Published Online: 2015-6-30
Published in Print: 2015-7-1

© 2015 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am-2015-4966
Keywords for this article
Sillimullite; new mineral; crystal structure; electron microprobe; sillimanite; mullite

Articles in the same Issue

  1. Highlights and Breakthroughs. Going small: Nanoscale geochronology using atom probe tomography
  2. Highlights and Breakthroughs. Bubbles do matter!
  3. Outlooks in Earth and Planetary Materials. Assuring the future of mineralogy
  4. Review. NMR and computational molecular modeling studies of mineral surfaces and interlayer galleries: A review
  5. Presidential Address. Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals
  6. 222Rn and 220Rn emanations as a function of the absorbed α-doses from select metamict minerals
  7. Compositional variation and timing of aluminum phosphate-sulfate minerals in the basement rocks along the P2 fault and in association with the McArthur River uranium deposit, Athabasca Basin, Saskatchewan, Canada
  8. Kinetics of deuteration in andradite and garnet
  9. Aluminum and iron behavior in glasses from destabilized spinels: A record of fluid/meltmineral interaction in mantle xenoliths from Massif Central, France
  10. Elastic plastic self-consistent (EPSC) modeling of plastic deformation in fayalite olivine
  11. Sc- and REE-rich tourmaline replaced by Sc-rich REE-bearing epidote-group mineral from the mixed (NYF+LCT) Kracovice pegmatite (Moldanubian Zone, Czech Republic)
  12. Inter-laboratory comparison of fission track confined length and etch figure measurements in apatite
  13. Origin and significance of the yellow cathodoluminescence (CL) of quartz
  14. Synthesis of large and homogeneous single crystals of water-bearing minerals by slow cooling at deep-mantle pressures
  15. A new mineral from the Bellerberg, Eifel, Germany, intermediate between mullite and sillimanite
  16. Yusupovite, Na2Zr(Si6O15)(H2O)3, a new mineral species from the Darai-Pioz alkaline massif and its implications as a new microporous filter for large ions
  17. Transition metal incorporation into mackinawite (tetragonal FeS)
  18. Pink color in Type I diamonds: Is deformation twinning the cause?
  19. Effect of crystal defects on diamond morphology during dissolution in the mantle
  20. Silica polymorphs in lunar granite: Implications for granite petrogenesis on the Moon
  21. Identification of nanocrystalline goethite in reduced clay formations: Application to the Callovian-Oxfordian formation of Bure (France)
  22. Dependence of R fluorescence lines of rubies on Cr3+ concentration at various temperatures, with implications for pressure calibrations in experimental apparatus
  23. Topotactic and reconstructive changes at high pressures and temperatures from Cs-natrolite to Cs-hexacelsian
  24. High-pressure equation of state and phase transition in PbAl2Si2O8 feldspar
  25. Enthalpies of formation of rare earth niobates, RE3NbO7
  26. Weathering of galena: Mineralogical processes, hydrogeochemical fluid path modeling, and estimation of the growth rate of pyromorphite
  27. Dissolved Cl, oxygen fugacity, and their effects on Fe behavior in a hydrous rhyodacitic melt
  28. The mobility of Nb in rutile-saturated NaCl- and NaF-bearing aqueous fluids from 1–6.5 GPa and 300–800 °C
  29. The effect of the [Na/(Na+K)] ratio on Fe speciation in phonolitic glasses
  30. Technical Note: Calculation of stoichiometry from EMP data for apatite and other phases with mixing on monovalent anion sites
  31. The effects of immobilized carboxylic-functional groups on the dynamics of phase transformation from amorphous to octacalcium phosphate
  32. Letter. Structural and chemical characterization of Mg[(Cr,Mg)(Si,Mg)]O4, a new post-spinel phase with sixfold-coordinated silicon
  33. Letter. Partition of Al between Phase D and Phase H at high pressure: Results from a simultaneous structure refinement of the two phases coexisting in a unique grain
  34. Letter. The speciation of carbon monoxide in silicate melts and glasses
  35. Letter. Lightning-induced shock lamellae in quartz
  36. New Mineral Names
  37. Book Review
  38. Errata
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