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Excess functions of chlorite solid solutions and neoformation of Fe-chlorites; some implications of recent thermochemical measurements

Stephen Aja
Excess functions of chlorite solid solutions and neoformation of Fe-chlorites; some implications of recent thermochemical measurements
American Mineralogist (February 2019) 104 (2): 232-243

Abstract

A full complement of standard state thermodynamic properties (Delta (sub f) G (sub 298.1) degrees , Delta (sub a) G (sub T,i) (super o) , S (sub 298.1) (super o) , and C (sub P) (super o) ) has been determined for a magnesian chamosite [Fe-Chl(W)] and a ferroan clinochlore [Mg-Chl] investigated by calorimetry and low-temperature hydrothermal experiments; this makes these two samples the only natural chlorites whose complete set of thermochemical properties have been reported. Delta (sub f) G (sub 298.1) (super o) for Mg-Chl and Fe-Chl (W) have been determined to be -8161.76 + or - 32.50 and -7278.97 + or - 21.50 kJ/mol, respectively. Ternary molecular chlorite solid solution modeling approaches have been developed for Al-rich and Si-rich chlorites; unlike available atomic site-mixing chlorite solid-solution models, a molecular model obviates the need for the adoption of a putative structural chemistry. The calculated excess entropy of mixing in the ternary system exhibits a curvilinear dependence on composition and at 25 degrees C, G (sub ss) (super ex) vary from about -72 to 413 kJ/mol implying a significant deviation from ideality. The effect of di-trioctahedral substitutions was evaluated by modeling the solid solutions in the quaternary amesite-chamosite-clinochlore-sudoite system for aluminous chlorites; excess functions (S (super ex) , G (super ex) ) calculated for these quaternary and ternary solid solutions are marginally different, inherently validating the ternary model. The molecular solid solution model further unmasks significant deficiencies in the available database of standard state thermodynamic properties of chlorites. Finally, pursuant to the recent recognition that green rusts probably play significant roles in the cycling of iron through sedimentary sequences, the neoformation of authigenic iron chlorites from green rusts has been examined; green rusts will readily transform to berthierine and Fe-chlorites except under oxidizing conditions atypical of aquatic environments and ferrugineous sediments.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 104
Serial Issue: 2
Title: Excess functions of chlorite solid solutions and neoformation of Fe-chlorites; some implications of recent thermochemical measurements
Author(s): Aja, Stephen
Affiliation: City University of New York, Brooklyn College, Department of Earth & Environmental Sciences, Brooklyn, NY, United States
Pages: 232-243
Published: 201902
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 58
Accession Number: 2019-044208
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201923
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