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Use of TEM‐EDX for structural formula identification of clay minerals: a case study of Di Linh bentonite, Vietnam
Author(s) -
Hoang-Minh Thao,
Kasbohm Jörn,
Nguyen-Thanh Lan,
Nga Pham Thi,
Lai Le Thi,
Duong Nguyen Thuy,
Thanh Nguyen Duc,
Thuyet Nguyen Thi Minh,
Anh Dao Duy,
Pusch Roland,
Knutsson Sven,
Ferreiro Mählmann Rafael
Publication year - 2019
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s1600576718018162
Subject(s) - bentonite , illite , clay minerals , montmorillonite , vermiculite , kaolinite , structural formula , mineralogy , fourier transform infrared spectroscopy , chlorite , dissolution , geology , powder diffraction , chemistry , mineral , materials science , analytical chemistry (journal) , crystallography , chemical engineering , metallurgy , environmental chemistry , composite material , geotechnical engineering , alkyl , organic chemistry , quartz , engineering
Transmission electron microscopy linked with energy‐dispersive X‐ray spectroscopy (TEM‐EDX) was applied to characterize mineralogical signals of weathering processes in the Di Linh bentonite deposit (Vietnam) and to visualize the effects of Na activation on the smectitic phases. Modelling of X‐ray diffraction patterns (oriented mount) was applied in order to refine the computed structural formula. X‐ray diffraction, X‐ray fluorescence and Fourier‐transform infrared spectroscopy (FT‐IR) methods were also applied to verify the TEM‐EDX results. An Excel ‐based routine has been developed in this research to allow fast computation of structural formulae and classification of the investigated clay particles. This routine supports the acquirement of 100–300 TEM‐EDX analyses as a representative set of individual particles for each sample. The Excel ‐based routine involves end members of different clay‐mineral groups and interstratifications with two or three members ( e.g. illite–smectite interstratifications – IS‐ml; dioctahedral vermiculite–smectite interstratifications – diVS‐ml; and kaolinite–montmorillonite–dioctahedral vermiculite interstratifications – KSV‐ml). The routine is now freely available. According to the identification procedure, the <2 µm fraction of the Di Linh bentonite (Vietnam) is composed mainly of K‐ and charge‐deficient illite–smectite interstratifications (or diVS‐ml): montmorillonite‐rich randomly ordered (R0) type and illite‐rich regularly ordered (R1) type. Additionally, Fe‐poor KSV‐ml was identified. Industrial Na activation of the Di Linh bentonite resulted in an increase of the R1 diVS‐ml portion and dissolution of a large part of the smectite‐rich phases. The TEM‐EDX approach also gave analytical proof of a sedimentary process for Di Linh smectite. The parent muscovite was altered in two different environments: (i) K‐leaching and layer‐wise alteration into kaolinite (weathering), and (ii) further edge‐controlled alteration of mica into lath‐like montmorillonite particles associated with a dissolution of kaolinite layers from the former kaolinite–mica intergrowths by heat impact (basalt flow).