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The structure of (Ca,Co)CoSi (sub 2) O (sub 6) pyroxenes and the Ca-M (super 2+) substitution in (Ca,M (super 2+) )M (super 2+) Si (sub 2) O (sub 6) pyroxenes (M (super 2+) = Co, Fe, Mg)

Luciana Mantovani, Mario Tribaudino, Francesco Mezzadri, Gianluca Calestani and Geoffrey Bromiley
The structure of (Ca,Co)CoSi (sub 2) O (sub 6) pyroxenes and the Ca-M (super 2+) substitution in (Ca,M (super 2+) )M (super 2+) Si (sub 2) O (sub 6) pyroxenes (M (super 2+) = Co, Fe, Mg)
American Mineralogist (July 2013) 98 (7): 1241-1252

Abstract

The crystal structure of three C2/c clinopyroxenes with composition (Ca (sub 0.8) Co (sub 0.2) )CoSi (sub 2) O (sub 6) , (Ca (sub 0.6) Co (sub 0.4) )CoSi (sub 2) O (sub 6) and (Ca (sub 0.4) Co (sub 0.6) )CoSi (sub 2) O (sub 6) was refined down to R (sub 4sigma ) = 2.6% by single-crystal X-ray diffraction. The crystals were synthesized at P = 3 GPa by cooling from 1500 to 1200 degrees C in a piston-cylinder apparatus. At the end of the refinement cycles, electron density residuals (up to 2.1 e (super -) ) were observed close to the M2 site and related to the site splitting of Ca and Co in the M2 polyhedron in the two subsites M2 and M2'. Split refinement significantly improved the agreement factor and decreased the uncertainty in the atomic coordinates. Similar features were found in (Ca,Mg)MgSi (sub 2) O (sub 6) and (Ca,Fe)FeSi (sub 2) O (sub 6) intermediate pyroxenes. The average structural changes related to the cation substitution at the M2 site in (Ca,Co)CoSi (sub 2) O (sub 6) , (Ca,Mg)MgSi (sub 2) O (sub 6) , and (Ca,Fe)FeSi (sub 2) O (sub 6) pyroxenes are similar: the T tetrahedron becomes more regular, the difference between M1-O bond lengths increases, and the M2-O3 bond lengths with the furthermost O3 oxygen atoms become longer. The changes in the M2-O bond distances are, however, not linear, and they are higher for more increased substitution. The largest structural deformation occurs on the (010) plane, with higher deformation at about 60 degrees from the c axis for any composition. The orientation of the deformation ellipsoid is most related to a shift in tetrahedral chains. The scalar deformation for the cation substitution, epsilon (sub s) , is linearly related to the cation radius of the average M2 site (IR (super M2) ), i.e., the deformation is higher as the cation size decreases, following the equation: epsilon (sub s) = -0.0072(12)IR (super M2) + 0.0082(13), R (super 2) = 0.75. Increasing deformation with cation substitution is supported as the major limiting factor for solid solution. The displacement parameters for unsplit M2, O2, and O3 atoms increase up to the intermediate composition, indicating a local configuration for the M2 polyhedron centered by Ca and Co. However no significant change in U (sub eq) of the O3 atom is observed up to 20% substitution of the smaller cation in the M2 site. Comparison with Raman spectral data suggests that local chain structural configurations occur only for the substitution of the smaller cation in the M2 site higher than 20%, and that the substitution mechanism is different for C2/c clinopyroxenes with lower and higher Ca content.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 7
Title: The structure of (Ca,Co)CoSi (sub 2) O (sub 6) pyroxenes and the Ca-M (super 2+) substitution in (Ca,M (super 2+) )M (super 2+) Si (sub 2) O (sub 6) pyroxenes (M (super 2+) = Co, Fe, Mg)
Affiliation: Universita di Parma, Dipartimento di Fisica e Scienze della Terra "Macedonio Melloni", Parma, Italy
Pages: 1241-1252
Published: 201307
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 41
Accession Number: 2013-061396
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
Secondary Affiliation: University of Edinburgh, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201337
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