In Brazil, the largest bentonite deposits, located in the Boa Vista region of Paraíba State, have been studied extensively. However, with the near depletion of high-quality varieties from this region, new deposits are being explored elsewhere in the Brazilian northeast. Among these, the Formosa deposit, in Maranhão State, stands out. In this study, the <0.1 μm clay fraction highly enriched in smectite from the Formosa deposit was characterized and compared with three commercial smectites (Bofe, Chocolate, and Verde Lodo) from the Boa Vista region. Analytical techniques included X-ray diffraction, infrared spectroscopy, transmission electron microscopy, and atomic absorption spectroscopy. Although previously described as montmorillonites, their crystal chemistry has not yet been elucidated fully. The present study therefore aimed to clarify the crystal chemistry of these smectites by resolving the distribution of layer charge between tetrahedral and octahedral sheets and evaluating its implications for swelling behavior. The results revealed that these Al-rich smectites also exhibit a significant tetrahedral layer charge. Indeed, they possess sufficient tetrahedral charge (~0.15 per half unit cell) to induce interlayer swelling after the Hofmann-Klemen effect, a value lower than that commonly reported for natural smectites. The structural and chemical similarities between Formosa and the three commercial smectites demonstrate that the Formosa deposit is a promising alternative source to the smectites from Boa Vista.