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GEOREF RECORD

Al, Si, and Mg occupancies in tetrahedrally and octahedrally coordinated sites in synthetic aluminous tremolite

David M. Jenkins, Barbara L. Sherriff, Janice Cramer and Zhi Xu
Al, Si, and Mg occupancies in tetrahedrally and octahedrally coordinated sites in synthetic aluminous tremolite
American Mineralogist (April 1997) 82 (3-4): 280-290

Abstract

A series of aluminous tremolites was synthesized at 800-850 degrees C and 6-13 kbar along the join Ca (sub 1.8) Mg (sub 5.2) Si (sub 8) O (sub 22) (OH) (sub 2) - Ca (sub 1.8) (Mg (sub 4.2) Al) (AlSi (sub 7) )O (sub 22) (OH) (sub 2) by means of the Mg-Tschermaks substitution. Although the amphibole yields were high (97-99 wt%), traces of orthopyroxene, clinopyroxene, amorphous silica, and, for the most aluminous mixture, corundum, were identified in some synthesis products. The samples were characterized by electron microprobe (EMP). X-ray diffraction Rietveld structure refinement (XRD), Fourier-transform infrared (FTIR) spectroscopy, and magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy for the purpose of identifying the extent of Al, Mg, and Si order-disorder in the octahedral and tetrahedral sites. Both EMP and (super 27) Al NMR verify that the Mg-Tschermaks substitution, with even distribution of Al between tetrahedral and octahedral sites, is obeyed in these amphiboles up to a limit of 1.9 total Al atoms per formula unit (apfu). (super 29) Si MAS NMR spectra indicate that Al substitutes essentially equally into both the T1 and T2 sites. For octahedral sites, the FTIR spectra indicate the presence of Al on the M1, M3, or both sites (presently indistinguishable) by the presence of an (MgMgAl)-OH band at 3652 cm (super -1) , which increases steadily with increasing A1 content. The FTIR and (super 27) Al MAS NMR spectra show that A1 is always present in the M1-M3 sites as well as the M2 site but partitioned among these sites in ways that are not clearly resolved at present. It is clear in this study that A1 is more widely distributed over the octahedral and tetrahedral sites in synthetic aluminous tremolite with only a moderate Al content (<2.0 Al apfu) than has been traditionally thought.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 82
Serial Issue: 3-4
Title: Al, Si, and Mg occupancies in tetrahedrally and octahedrally coordinated sites in synthetic aluminous tremolite
Affiliation: Binghamton University, Department of Geological Sciences and Environmental Studies, Binghamton, NY, United States
Pages: 280-290
Published: 199704
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 40
Accession Number: 1997-042331
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: 6 tables
Secondary Affiliation: University of Manitoba, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 199715
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