Co-precipitation of primary dolomite and Mg-rich clays in Deep Springs Lake, California

Abstract

In contrast to the prevalence of dolomite [CaMg(CO₃)₂] in the geological record, there are few instances of recent formation. This discrepancy occurs despite supersaturation with respect to dolomite in many modern marine and lacustrine environments. Additionally, laboratory experiments have struggled to precipitate dolomite at ambient temperatures (<40°C) even under highly saturated conditions. However, recent work has found dissolved silica to be an effective catalyst for the direct precipitation of dolomite. To test this hypothesis, the hydrology and mineralogy of Deep Springs Lake, a playa setting with primary dolomite precipitation has been explored. In this study, the central playa sites with the highest sedimentation rates were found to have dissolved silica concentrations close to saturation. Rietveld refinement showed that bulk mineral assemblages of core samples contain 20 to 40% fine-grained, partially ordered dolomite along with a 10 to 20% poorly crystalline clay fraction, while transmission electron microscopy imaging found a co-precipitation relationship between the fine-grained dolomite and Mg-rich smectite clays, where the Mg-rich smectite clays act as a sink for the excess dissolved silica during evaporation. Transmission electron microscopy results indicate that the nanodolomite crystals formed through surface-induced nucleation and growth processes in the presence of dissolved silica as a catalyst. The broad coincidence of conditions for the direct precipitation of dolomite and Mg-rich smectite clays provides evidence that silica may be a key to the dolomite problem, as well as constraining palaeoenvironments such as the Pre-salt Barra Velha Formation.