For more than forty years, The Clay Minerals Society has dispensed a set of source clays which have enabled a large number of researchers to work on similar materials. Many of these source clays remained unchanged over the years but, conversely, other clays have gone out of stock and thus were replaced. This was the fate of montmorillonite STx-1a, which was replaced by STx-1b. Although STx-1a and STx-1b share many basic chemical and mineralogical features, some minor differences exist that can affect behavior. A baseline characterization of the source clay STx-1b, which was the objective of this study, was, therefore, necessary to provide researchers a tool useful not only for new investigation but also to compare new results obtained on STx-1b with literature data on STx-1a. This characterization was gained using traditional and advanced methods that included: 1) chemical composition (major and trace elements); 2) cation exchange capacity determination; 3) thermal analyses coupled with evolved gas mass spectrometry; 4) quantitative mineralogical characterization using powder X-ray diffraction and Rietveld-RIR (Reference Intensity Ratio) refinement; 5) X-ray absorption spectroscopy at the Fe K-edge; 6) diffuse reflectance ultraviolet-visible and infrared spectroscopies; and 7) 29Si, 27Al, and 1H magic-angle spinning nuclear magnetic resonance measurements. According to this multi-analytical approach, the chemical formula for STx-1b is (Si7.753Al0.247) (Al3.281Mg0.558Fe0.136Ti0.024Mn0.002) (Ca0.341Na0.039 K0.061)O20(OH)4.