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Ti-rich andradites; chemistry, structure, multi-phases, optical anisotropy, and oscillatory zoning

Sytle M. Antao, Shaheera Mohib, Mashrur Zaman and Robert A. Marr
Ti-rich andradites; chemistry, structure, multi-phases, optical anisotropy, and oscillatory zoning
The Canadian Mineralogist (January 2015) 53 (1): 133-158

Abstract

Electron probe microanalyzer (EPMA) data were obtained for 11 Ti-rich andradite garnet, (super [8]) X (sub 3) (super [6]) Y (sub 2) (super [4]) Z (sub 3) (super [4]) O (sub 12) , samples from Magnet Cove, Arkansas, USA. The crystal structures of nine of these samples were refined with the Rietveld method, space group la(macron)3d, and monochromatic synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. One sample (MC9) contains two distinct compositions corresponding to different phases and subtle compositional differences are observed in two other samples (MC8 and MC10). The EPMA data falls on a straight line representing solid solutions from the endmember andradite, Ca (sub 3) Fe (super 3+) (sub 2) Si (sub 3) O (sub 12) , to a composition, Ca (sub 3) (Ti (super 4+) (sub 1.5) Fe (super 2+) (sub 0.5) )[Fe (super 3+) Si (sub 2) ]O (sub 12) , with the coupled substitution: Fe (super 3+) + Si (super 4+) <--> Ti (super 4+) (sub 1.5) + Fe (super 2+) (sub 0.5) . The Y site contains Ti (super 4+) cations (up to nearly equal 1.1 atom per formula unit, apfu). A few samples show oscillatory zoning that arises mainly from the distribution of Ti atoms coupled with subtle variations in Fe, Al, and Mg atoms. The HRPXRD results show that seven samples contain two or three cubic phases intergrown together. These seven samples exhibit birefringence because of strain arising from structural mismatch. There are three Ti-rich schorlomite samples (MC1, MC5, and MC7) of which MC1 and MC7 are single cubic phases and are isotropic. The bond distances (average <X-O>, Y-O, Z-O, and mean <D-O>) change linearly with increasing a unit-cell parameter. The Y-O distance is nearly constant. The Z-O distance increases most because of Fe (super 3+) (0.49 Aa) substituting for Si (super 4+) (0.26 Aa) cations at the Z site.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 53
Serial Issue: 1
Title: Ti-rich andradites; chemistry, structure, multi-phases, optical anisotropy, and oscillatory zoning
Affiliation: University of Calgary, Department of Geoscience, Calgary, AB, Canada
Pages: 133-158
Published: 201501
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 67
Accession Number: 2015-094134
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
N34°07'60" - N34°30'00", W93°25'00" - W92°40'00"
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201540
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