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Conversion of serpentine to smectite under hydrothermal condition; implication for solid-state transformation

Ji Shichao, Zhu Jianxi, He Hongping, Tao Qi, Zhu Runliang, Ma Lingya, Chen Meng, Li Shangying and Zhou Junming
Conversion of serpentine to smectite under hydrothermal condition; implication for solid-state transformation
American Mineralogist (February 2018) 103 (2): 241-251

Abstract

Understanding clay mineral transformation is of fundamental importance to grasping phyllosilicate crystal chemistry and unraveling geochemical processes. In this study, hydrothermal experiments were conducted on lizardite and antigorite, to investigate the possibility of the transformation from serpentine to smectite, the effect of precursor minerals' structure on the transformation and the transformation mechanism involved. The reaction products were characterized using XRD, TG, HRTEM, and (super 27) Al MAS NMR. The results show that both lizardite and antigorite can be converted to smectite, but such conversion is much more difficult than that of kaolinite group minerals. The successful transformation is mainly evidenced by the occurrence of the characteristic (001) reflection of smectite at 1.2-1.3 nm in the XRD patterns and smectite layers with a thickness of 1.2-1.3 nm in HRTEM images of hydro-thermal products as well as the dehydroxylation of the newly formed smectite at a higher temperature in comparison to that of the starting minerals. The difficulty for the transformation of serpentine to smectite may be due to the lack of enough available Al in the reaction system, in which the substitution of Al (super 3+) for Si (super 4+) in the neo-formed tetrahedral sheet is critical to control the size matching between the neo-formed tetrahedral sheet and octahedral sheet in starting minerals. Since the neighboring layers in antigorite are linked by the strong Si-O covalent bonds, the transformation only takes place at the edges of an antigorite layer rather than the whole layer, and the neo-formed smectite is non-swelling due to the inheritance of such Si-O covalent bonds. The conversion of lizardite to smectite is more feasible than that of antigorite, accompanied by exfoliation. This leads to a prominent decrease of the particle size in the hydrothermal products and the number of phyllosilicate layers contained therein. Two dominant pathways were observed for the transformation of lizardite and antigorite into smectite, i.e., conversion of one serpentine layer to one smectite layer via attachment of Si-O tetrahedra onto the octahedral sheet surface of the starting minerals and two adjacent serpentine layers merging into one smectite layer. In the case of the latter, dissolution of octahedra and inversion of tetrahedral sheets took place during the transformation. Besides these two dominant pathways, precipitation and epitaxial growth of smectite were also observed in the cases of lizardite and antigorite, respectively. The present study suggests that solid-state transformation is the main mechanism for conversion of serpentine minerals to smectite, similar to the transformation of kaolinite group minerals to beidellite.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 103
Serial Issue: 2
Title: Conversion of serpentine to smectite under hydrothermal condition; implication for solid-state transformation
Affiliation: Chinese Academy of Sciences, Laboratory of Mineralogy and Metallogeny, Guangzhou, China
Pages: 241-251
Published: 201802
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 78
Accession Number: 2018-033562
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: University of Chinese Academy of Sciences, CHN, China
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201808
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