Rhyolitic obsidian was reacted with natural waters to study the effect of water chemistry and biological activity on the composition and formation mechanisms of clay. Two sets of experiments (18 months, 6 years) used fresh, hypersaline water (Mg-Na-SO4-Cl- and NaCl-rich) and seawater. The 6-year experiments produced the transformation of obsidian into quartz, apparently by in situ re-crystallization (Cuadros et al., 2012). The most abundant neoformed clay was dioctahedral (typically montmorillonite), indicating chemical control by the glass (where Al > Mg). Altered glass morphology and chemistry in the 18-months experiments indicated in situ transformation to clay. Magnesium-rich (saponite) clay formed under water-chemistry control in the bulk and within biofilms with elevated Mg concentration (Cuadros et al., 2013). The contact between microbial structures and glass was very intimate. Glass transformation into quartz may be due to some characteristic of the obsidian and/or alteration conditions. Such combination needs not to be uncommon in nature and opens new possibilities of quartz origin.

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Purpose Mineral dust pollution is a concern for human health due to the reaction of mineral particles in the organism and their role as pathogen carriers. Human activity generates unconsolidated sediments that become a dust source. This study investigates the effect of microbial growth on dust stabilization through aggregation in order to help alleviate this problem. Materials and methods Four representative potential dust sources (volcanic ash, carbonate, mine waste, and diatomaceous earth) and three organic nutrients (humic acids, glucose+peptone and yeast extract) were selected. We used the indigenous microbial communities in the mineral samples, rain and tap water. All experiments were carried out in Petri dishes, in moist conditions. The experiments were illuminated artificially for 12 h a day and control experiments were carried out in the dark. Results and discussion Biological growth occurred within a few days, and followed the same sequence (bacteria, fungi, and green algae) irrespective of rock type or nutrient. After weeks, the biological development was large and dominated by the algae. Bacteria and fungi developed in some control experiments. The microbial mats successfully aggregated the sediment and formed large patches that remained stable after complete drying. Tests to measure organic C content may indicate humic acid as the most efficient of the three nutrients. SEM observation showed intricate interlacing between mineral grains and microbial structures, and the predominance of large algal mats. Measurements of particle size in water using laser granulometry showed larger particles after the experiments, despite the aqueous medium having a disaggregating effect. Simulated wind erosion indicated the removal of lower proportions of the sediment in the experiments than in their controls. Conclusions Our results suggest that rapid stabilization of the surface of disaggregated sediments using microbial growth is feasible, the microbial mat remains biologically active in moist conditions and the dry sediment can be physically stable for a period of time unless disrupted mechanically. Algae have the maximum aggregating effect. This system of rapid stabilization appears to be equally feasible in sediments of very different mineralogical and chemical composition.