HYDROUS SODIUM SILICATE MINERALS AND DIAGENETIC DERIVATIVES FROM LAKE MAGADI, KENYA

Na-silicate minerals such as magadiite are rare minerals found in unique geologic settings like Lake Magadi in southern Kenya, characterized by extreme alkalinity and high silica activity. Understanding the genesis and diagenesis of these Na-silicates is important in interpreting Quaternary environmental change in the region. Magadiite is a hydrated phyllosilicate mineral with a chemical composition of Na₇Si₁₆O₈(OH)₈ · 2(H₂O). Kenyaite is an associate mineral of magadiite, with a chemical formula of Na₂Si₂₂O₄₁(OH)₈ · 6(H₂O). Montmorillonite and the zeolite analcime are also associated with magadiite deposits and likely formed through related processes. In contrast to the Na-silicates, zeolites have a tectosilicate 3-dimensional framework that hosts aluminum; e.g., analcime: Na₂(Al₂Si₄O₁₂) · 2(H₂O). These minerals likely formed as chemical precipitates in alkaline brines or from Na-Al-Si gels that coalesced around springs near the lake margin and were subsequently washed into the basin. Magadi-type cherts are thought to form from precursor Na-silicates and opal A at or below the sediment-water interface; microbial activity may also be involved. In contrast, Na-silicates dehydrate rapidly upon subaerial exposure, resulting in the alteration of these minerals and the precipitation of diagenetic derivatives. Fresh outcrop samples of magadiite were collected during the 2015 field season in southern Kenya. Samples were shipped and arrived in their hydrated state, thus allowing for dehydration experiments via XRD analysis. Aliquots were air-dried and dried in an oven for 24 hours at 50°C to analyze the dehydrated state. Hydrated magadiite samples were analyzed in their hydrated state to characterize the composition prior to dehydration. The same hydrated sample was then analyzed repeatedly in ten minute intervals for 12 hours to record the crystallographic changes that occur during dehydration at room temperature and humidity. As magadiite dehydrated, derivative minerals formed including quartz (chert precursors?), trona, and halite. These results suggest that although magadiite likely formed frequently throughout the basin’s history, it is poorly preserved in the stratigraphic record due to alteration to chert in groundwater or through dehydration upon subaerial exposure.