New insights into the crystal chemistry of sauconite (Zn-smectite) from the Skorpion zinc deposit (Namibia) via a multi-methodological approach

Abstract

A multi-methodical characterization of a sauconite (Zn-bearing trioctahedral smectite) specimen from the Skorpion ore deposit (Namibia) was performed by combining X-ray powder diffraction (XRPD), cation exchange capacity (CEC) analysis, differential thermal analysis (DTA), thermo-gravimetry (TG), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM-HRTEM-AEM). The X-ray diffraction pattern exhibits the typical features of turbostratic stacking disorder with symmetrical basal 00l reflections and long-tailed hk bands, as confirmed by TEM observations. Besides sauconite, the sample contains minor amounts of kaolinite, dioctahedral smectite, and quartz. CEC analysis provides a total of Ca (~69%), Mg (~26%), Na (~4%), and K (0.7%) exchangeable cations. Therefore, Zn is located exclusively within the octahedral site of sauconite. TG analysis of the sample yields a total mass loss of about 17%. Three endothermic peaks can be observed in the DTA curve, associated with dehydration and dehydroxylation of the material. An exothermic peak at 820 °C is also present as a consequence of decomposition and recrystallization. The infrared spectrum shows the typical Zn3OH stretching signature at 3648 cm–1, whereas, in the OH/H2O stretching region two bands at 3585 and 3440 cm–1 can be attributed to stretching vibrations of the inner hydration sphere of the interlayer cations and to absorbed H2O stretching vibration, respectively. Diagnostic bands of kaolinite impurity at ~3698 and 3620 cm–1 are also found, whereas 2:1 dioctahedral layer silicates may contribute to the 3585 and 3620 cm–1 bands. Finally, using the one-layer supercell approach implemented in the BGMN software, a satisfactory XRPD profile fitting model for the Skorpion sauconite was obtained. These findings have implications not only for economic geology/recovery of critical metals but also, more generally, in the field of environmental sciences.

Local structure determination of Zn-smectite

“…It was probably because there are no other cations available with the exception of Na + , during the formation of S-Zn4. This is obviously different from the structure of sauconite, in which Zn is mainly located within the octahedral site, and the Zn 2+ adsorbed at the mineral surface or fixed in the interlayer is negligible or lacking (Schingaro et al, 2021).…”

“…Typical flaky grains with curled ribbon-like edges were observed in TEM images for all products (Figure 4). This is a very common morphology for smectite minerals, such as saponite and sauconite (Guven, 1988, Schingaro et al, 2021, Tao et al, 2019.…”

“…Zinc-rich smectites (Zn-smectites) are associated with supergene nonsulfide ores worldwide (Mondillo et al, 2015). Some of these are predominant economic minerals (e.g., sauconite) (Schingaro et al, 2021) of the mineral assemblage derived by weathering (Newman andBrown, 1987, Ross, 1946). As natural two-dimensional (2D) inorganic materials, Zn-smectites have wide applications as catalysts and catalyst supports, luminescent materials, adsorbents, etc.…”