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Cation ordering, twinning, and pseudo-symmetry in silicate garnet; the study of a birefringent garnet with orthorhombic structure

Huifang Xu, Shiyun Jin, Seungyeol Lee and Philip E. Brown
Cation ordering, twinning, and pseudo-symmetry in silicate garnet; the study of a birefringent garnet with orthorhombic structure
American Mineralogist (March 2023) 108 (3): 572-583

Abstract

The crystal structure of a birefringent garnet ( approximately Adr (sub 53) Grs (sub 47) ) that occurs as a late-stage rim on andradite from Stanley Butte, Graham County, Arizona is analyzed and refined using single-crystal XRD. The structure has an orthorhombic I 2/a 1 2/d (unconventional setting for Fddd) space group symmetry, with unit-cell parameters of a = b = 11.966(3) Aa, c = 11.964(3) Aa , alpha = beta = 90 degrees , gamma = 90.29(2) degrees , V = 1713.0(7) Aa (super 3) , Z = 8. The orthorhombic garnet displays very high birefringence (delta approximately 0.021) produced by the strong Fe-Al ordering in the octahedral sites, with Fe occupancies of 0.804 and 0.221 in Y (sub 1) and Y (sub 2) sites, respectively. Diffraction peaks (such as 101 and 103) violating the Ia3d symmetry of cubic garnet are obvious even in powder XRD pattern. The homogenization temperatures of the fluid inclusions suggest that the low-crystallization temperature is responsible for the ordered orthorhombic structure. The strong ordering state of the structure and the sharp boundaries in the chemical zoning in the crystal (between approximately Adr (sub 53) Grs (sub 47) and approximately Adr100) indicate the orthorhombic intermediate grandite garnet is a thermodynamically stable phase at low temperature, separated by wide miscibility gaps from the pure end-members (grossular and andradite) with cubic structures. Most of the previously reported triclinic garnet structures are likely artifacts produced by pseudo-merohedral twinning of less-ordered orthorhombic structure, as indicated by the characteristic pairing pattern of different Y-sites with the same occupancies.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 108
Serial Issue: 3
Title: Cation ordering, twinning, and pseudo-symmetry in silicate garnet; the study of a birefringent garnet with orthorhombic structure
Affiliation: University of Wisconsin Madison, Department of Geoscience, Madison, WI, United States
Pages: 572-583
Published: 20230301
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 74
Accession Number: 2023-030036
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2023, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2023
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