Transformation of 1:1 type clay minerals into smectite under hydrothermal condition. H. He et al.

Transformation of 1:1 type clay minerals into smectite under hydrothermal condition
Hongping He*, Shichao Ji, Shangying Li, Qi Tao, & Jianxi Zhu

Understanding clay mineral transformation is of importance to unraveling geological processes and to utilizing clay mineral resources. So far, two pathways have been identified, i.e., the transformation between 2:1 type
clay minerals (Stixrude and Peacor, 2002) and from 2:1 type to 1:1 type (Altschuler et al., 1963). However, the conversion of 1:1 to 2:1 type has rarely been reported. In this study, we demonstrate that 1:1 type clay minerals can transform into 2:1 smectite under hydrothermal condition (He et al., 2017; Ji et al., 2018). XRD patterns of the hydrothermal products display characteristic basal spacing of smectite group minerals at 1.2 - 1.3 nm, consistent with HRTEM observations. The consumption of surface OH in precursor minerals during the transformation led to a dramatic decrease of mass loss of dehydroxylation in TG curves. Note that the transformation from kaolinite and halloysite to smectite was much easier than that from serpentine to smectite. The difficulty for the transformation of serpentine may be due to the lack of enough available Al in the reaction system, in which the substitution of Al3+ for Si4+ 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. For kaolinite, halloysite and lizardite, exfoliation occurred during their conversions, leading to a prominent decrease in the number of phyllosilicate layers contained
in the hydrothermal products. However, the transformation could only take place at the edges of an antigorite layer rather than the whole one, due to the strong Si-O covalent bonds between adjacent layers in antigorite.
The inheritance of such Si-O covalent bonds also resulted in non-swelling property of the neo-formed smectite from antigorite. Two main pathways were proposed for the transformation of 1:1 type clay minerals into
smectite: 1) conversion of one 1:1 type layer to one smectite layer via attachment of Si-O tetrahedra onto the octahedral sheet surface of the starting minerals and 2) two adjacent 1:1 type layers merging into one
smectite layer. The present study suggests that solid state transformation is the main mechanism for conversion of 1:1 type clay minerals to smectite.