Primary and secondary deposits in the Tan Huong-Truc Lau area of northern Vietnam's Yen Bai Province have supplied rubies (especially star rubies) and some sapphires-mostly of cabochon quality-for more than a decade. The gems are typically translucent to semitransparent and pink to purplish or brownish red, with rare color zoning. The most distinctive features of this corundum after polishing include growth zoning and asterism. The samples contain relatively high amounts of Fe and variable Ti and Cr. The geologic origin and gemological properties of this corundum are distinct from that of the adjacent Khoan Thong-An Phu area. For more than 20 years, Vietnam's Yen Bai Province, and the Luc Yen District in particular, have been widely known for producing highquality rubies and sapphires. Subsequent deposits found elsewhere in Vietnam (Long et al., 2004) include Quy Chau-Quy Hop (Nghe An Province), Di Linh (Lam Dong), Dak Ton (Dak Nong), and Ma Lam and Da Ban (Binh Thuan). Still, Yen Bai remains the country's most important source of ruby and sapphire. Much of the production consists of cabochonquality stones from the Tan Huong-Truc Lau area (e.g., figure 1), as described in this article.Vietnamese geologists first discovered gem-quality corundum and spinel at Luc Yen in 1983 (Vinh, 1991). In early 1987, the Geological Survey of Vietnam found abundant gem material in alluvium in Luc Yen's Khoan Thong area. Mining activity soon thrived, with many companies operating in the region (Voi, 1991). Other gem occurrences near Khoan Thong followed, such as Nuoc Ngap, Hin

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 Fouriertransform 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 mm 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).

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