Quartz dissolution associated with magnesium silicate hydrate cement precipitation

Lisa De Ruiter,Håkon Austrheim,Dag Kristian Dysthe,Anette Eleonora Gunnæs

doi:10.5194/se-12-389-2021

Abstract

Abstract. Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway. This occurs in deformed and recrystallized quartz grains deposited as glacial till covering part of the ultramafic body. Where the ultramafic body is exposed, weathering leads to high-pH (∼ 10), Mg-rich fluids. The dissolution rate of the quartz is about 3 orders of magnitude higher than experimentally derived rate equations suggest under the prevailing conditions. Quartz dissolution and cement precipitation start at intergranular grain boundaries that act as fluid pathways through the recrystallized quartz. Etch pits are also extensively present at the quartz surfaces as a result of preferential dissolution at dislocation sites. Transmission electron microscopy revealed an amorphous silica layer with a thickness of 100–200 nm around weathered quartz grains. We suggest that the amorphous silica is a product of interface-coupled dissolution–precipitation and that the amorphous silica subsequently reacts with the Mg-rich, high-pH bulk fluid to precipitate magnesium silicate hydrate cement, allowing for further quartz dissolution and locally a complete replacement of quartz by cement. The cement is the natural equivalent of magnesium silicate hydrate cement (M-S-H), which is currently of interest for nuclear waste encapsulation and for environmentally friendly building cement, but it has not yet been developed for commercial use. This study provides new insights that could potentially contribute to the further development of M-S-H cement.

Highlights

Weathering at the Earth’s surface leads to the breakdown of rocks and the release of chemical compounds to the weathering fluids and is an important process for the chemical cycle of elements and the chemistry of groundwater and soil
Hellmann et al (2012) concluded that chemical weathering of silicate minerals is controlled by nanoscale interfacial dissolution–precipitation mechanisms and proposed a continuum model for chemical weathering of silicates solely based on dissolution and reprecipitation
The quartz grains, which are the potential source of silica for the magnesium silicate hydrate cement, were studied in detail, and the results are listed below together with observations of the non-reacted initial quartz, i.e. the protolith

Summary

Introduction

Weathering at the Earth’s surface leads to the breakdown of rocks and the release of chemical compounds to the weathering fluids and is an important process for the chemical cycle of elements and the chemistry of groundwater and soil. The obtained weathering rates of silicate minerals from field samples can vary by multiple orders of magnitude from experimentally obtained rates, with the latter usually being higher (White et al, 2001; White and Brantley, 2003; Brantley, 2005; Zhu et al, 2006; Moore et al, 2012) This indicates the complexity of dissolution mechanisms in nature and the inability to measure dissolution rates under these complex conditions and timescales in the laboratory (Gruber et al, 2014).

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