open access

https://hdl.handle.net/2115/81951

Precipitation of Magnesium Silicate Hydrates in Natural Alkaline Surface Environments

英語

serpentinization

2020-06-30

一般社団法人 日本粘土学会

Clay Science

24(1)

1-13

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.

学術雑誌論文

Version of Record

0009-8574

2186-3555

AA00607148