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A calorimetric study of zoisite and clinozoisite solid solutions

Eugene A. Smelik, Gerhard Franz and Alexandra Navrotsky
A calorimetric study of zoisite and clinozoisite solid solutions
American Mineralogist (February 2001) 86 (1-2): 80-91

Abstract

Enthalpies of drop solution in molten lead borate have been measured for a series of four zoisite and 12 clinozoisite samples, spanning both epidote solid solutions. Mixing within the zoisite series between 0.0 and 0.2X (sub Al2Fe) appears to be ideal. The enthalpy of drop solution varies between 493.0+ or -3.2 and 496.0+ or -3.6 kJ/mol with a slight increase with increasing Fe content, based on a linear fit to the data. The clinozoisite solid solution shows more complex behavior. The enthalpy of drop-solution for samples between 0.28 and 0.65 X (sub Al2Fe) varies between 491.7+ or -4.2 and 500.7+ or -3.0 kJ/mol and shows significant scatter. For samples between 0.65 and 0.95 X (sub Al2Fe) , the drop solution enthalpies increase abruptly, going from 494.3+ or -3.4 to 521.1+ or -5.4 kJ/mol. end-member drop solution values for Fe-free zoisite were estimated by the linear fit and for clinozoisite by assuming a fictive Fe-free end-member, taking into account the estimated energy of the monoclinic-orthorhombic transition, and by extrapolation of the Fe-rich data to X (sub Al2Fe) = 1. Standard enthalpies of formation from the elements for Fe-free zoisite, Al-clinozoisite, and Fe-clinozoisite have been calculated based on the present data and data from the literature. The values are Delta H (super 0,zoi) (sub f,298.15) = -6878.5+ or -6.8 kJ/mol; Delta H (super 0.Al-czoi) (sub f,298.15) = -6882.5+ or -6.9 kJ/mol; Delta H (super 0.Fe-czo) (sub f,298.15) = -6461.9+ or -6.8 kJ/mol. The overall trend in the drop solution data reflects an apparent positive enthalpy of mixing for the clinozoisite series, consistent with the presence of a miscibility gap in the series between about 0.5 and 0.8 X (sub Al2Fe) . An attempt to fit the highly asymmetric enthalpy of mixing data with a two-parameter asymmetric Margules mixing model, however, failed to yield realistic values for the interaction parameters and the solvus closure temperature. The significant scatter in the data, between X (sub Al2Fe) = 0.0 and 0.65, may be due to short-range ordering, suggested by Fehr and Heuss-Assbichler (1997), or could be due to different states of metastable disorder in the samples. The steep increase in enthalpy going from X (sub Al2Fe) = 0.65 to 1.0 the may be due to the effect of possible increasing Fe (super 3+) -Al (super 3+) disorder among the M1 and M3 octahedral sites. The break in slope of the enthalpy data occurs at or near the composition where the onset of Fe (super 3+) disorder had been observed previously in clinozoisite.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 86
Serial Issue: 1-2
Title: A calorimetric study of zoisite and clinozoisite solid solutions
Affiliation: Quantum Catalytics, Fall River, MA, United States
Pages: 80-91
Published: 200102
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 63
Accession Number: 2001-073734
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Secondary Affiliation: Technische Universitaet Berlin, DEU, Federal Republic of GermanyUniversity of California at Davis, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200123
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