Use of TEM‐EDX for structural formula identification of clay minerals: a case study of Di Linh bentonite, Vietnam. Issue 1 (1st February 2019)

• Record Type: Journal Article
• Authors: 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
• Is Part Of: Journal of applied crystallography. Volume 52:Issue 1(2019)
• Page Start: 133
• Page End: 147
• Publication Date: 2019-02-01
• Subjects: clay minerals -- Di Linh bentonite -- transmission electron microscopy -- energy‐dispersive X‐ray spectroscopy -- structural formulae; Crystallography -- Periodicals; 548.05
• DOI: 10.1107/S1600576718018162
• Languages: English
• ISSNs: 0021-8898

Abstract

Abstract : A methodological approach is presented for computing the structural formula of clay minerals from transmission electron microscopy–energy‐dispersive X‐ray spectroscopy (TEM‐EDX) data of individual particles, including pure and complex interstratifications. The combination of TEM‐EDX and modelling of XRD patterns enables the refinement of the TEM‐EDX‐based structural formula. This methodology allows identification of the mineral composition and mineral transformations happening in both geological and industrial processes. Abstract : 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).