PRECIPITATION OF MAGNESIUM SILICATE HYDRATES IN NATURAL ALKALINE SURFACE ENVIRONMENTS

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Abstract

<p>Magnesium silicate hydrate (M–S–H) has been considered to play a significant role in different fields of engineering geology including radioactive waste disposal and geological storage of CO2. However, M–S–H has been discussed only with synthetic samples in most previous studies. To confirm and characterize M–S–H precipitated in natural surface environments, and to assess the formation conditions and processes of the M–S–H, we investigated present-day precipitation of M–S–H at an ultramafic body in the Kamuikotan tectonic belt, Hokkaido, Japan. We collected seepage, surface water, and surface sediments on the ultramafic rock mass. Seepage and surface water showed alkaline pH and Mg2+–HCO3− type water. Although bulk XRD analysis of the sediments did not clearly identify peaks of M–S–H due to its low crystallinity, microscopic observation showed that the sediments contained solid phases that are precipitated interstitially around the detrital serpentine particles. A TEM analysis identified the phases as M–S–H that may be mineralogically considered as a low-crystalline chrysotile with nano-tubular morphology. Thermodynamic calculations for the collected liquid samples suggest that mixed solution of seepage and surface water having high Si activity can induce the formation of M–S–H. The precipitation of M–S–H is likely to be a commonly occurring phenomenon in natural Mg–Si–H2O systems where geochemical environments contain alkaline fluid that shows high Si activity and meets the thermodynamic conditions for M–S–H formation. In addition to the precipitation of M–S–H in Mg–Si–H2O systems, we also observed that M–S–H is precipitated even under conditions including Ca and CO2 species (i.e., Mg–Ca–Si–CO2–H2O systems), with a simultaneous precipitation of aragonite. This study supports an understanding of the precipitation of M–S–H by mineral–water interactions in natural surface/subsurface environments for various engineering geology fields.</p>

Journal

  • Clay Science

    Clay Science 24 (1), 1-13, 2020-06-30

    The Clay Science Society of Japan

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