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Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation

Victor A. Drits, Douglas K. McCarty and Arkadiusz Derkowski
Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation
American Mineralogist (November 2012) 97 (11-12): 1922-1938


The <1 mu m fraction of a trans-vacant 1M illite (RM30) was studied by conventional and synchrotron X-ray diffraction (XRD) techniques, combined with thermogravimetric (TG, DTG) methods to investigate the structural transformation of illite at different temperatures and degrees of dehydroxylation (D (sub T) ). The oriented specimens preheated at 300 and 680 degrees C correspond to the non-dehydroxylated (D (sub T) =0) and completely dehydroxylated (D (sub T) =100%) 1 M illite structures. Deviation of the basal reflection positions from rationality, expressed by the coefficient of variation of d(00l) values, progressively increase from 0.05 at D (sub T) =4%, to 0.14 at D (sub T) =51%, and then decrease to 0.06 at D (sub T) =95%. Similarly, for each 00l reflection the full width at half-maximum (FWHM) shows a bell-like evolution with increasing preheating temperature. Both of these features are characteristic for mixed-layered structures. The experimental profiles of 00l reflections from the oriented partially dehydroxylated specimens perfectly matched the profiles from XRD pattern simulations calculated in terms of a mixed-layered structure in which the non-dehydroxylated (ND) and completely dehydroxylated (CD) illite layers are interstratified with a strong tendency to segregation. The content of the CD layers in the modeled mixed-layered structures of the preheated specimens show a significant linear correlation with the corresponding D (sub T) values (R (super 2) =0.99). In random powder XRD patterns collected with synchrotron radiation, the preheated specimens show a distinctive trend in the unit-cell parameters. However, the accuracy in determination of the unit-cell parameters at first decreases up to D (sub T) =61% and then increases with a further increase in D (sub T) . The evolution of FWHM values of individual hkl reflections is also similar to that observed for each 00l peak from oriented sample preparations. The unexpected evolution of the unit-cell parameters during progressive dehydroxylation is explained by the interstratification of ND and CD layers in illite. The formation of the mixed-layered structures during Al-rich 1M illite dehydroxylation is in agreement with the prediction from the kinetic model of partially dehydroxylated dioctahedral 2:1 clay structures where dehydroxylation of each portion of the initial OH groups corresponds to a zero-order reaction that is independent of the structural and chemical composition. The reaction is homogeneous and during partial dehydroxylation of the illite structure, ND layers transform into the CD layers without formation of an intermediate phase. A layer-by-layer dehydroxylation mechanism is suggested for thermally induced illite structural transformation.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 97
Serial Issue: 11-12
Title: Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation
Affiliation: Russian Academy of Science, Geological Institute, Moscow, Russian Federation
Pages: 1922-1938
Published: 201211
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 48
Accession Number: 2013-004023
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 7 tables
Secondary Affiliation: Chevron ETC, USA, United StatesPolish Academy of Sciences, Institute of Geological Sciences, POL, Poland
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: 201303
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