Formation of magnesium silicate hydrate cement in nature

Abstract

Tillite lithified by magnesium silicate hydrate (M-S-H) cement has been formed at the surface of the Feragen Ultramafic Body in SE Norway. Serpentinization of olivine-rich rocks led to formation of brucite that dissolves during weathering to form a high pH (>9) Mg-rich fluid. This fluid dissolves quartz deposited by glaciers during the Weichselian glaciation. Subsequent evaporation leads to the precipitation of a nanocrystalline magnesium silicate hydrate phase with the approximate composition Mg8Si8O20(OH)8•6H2O. The resulting cemented rock is characterized by disintegrated and partly dissolved quartz grains that are surrounded by the M-S-H cement. This process occurs at surface conditions in a subarctic climate and is known to take place on the timescale of a few decades, as constrained by mining activities. Magnesium-based cement is currently of high interest as the search for environmentally friendly cement is a pressing task considering that the widely used Portland cement accounts for about 7% of the worldwide anthropogenic CO2 emission. As this study provides new key insights into the formation process of natural CO2-neutral magnesium-based cement on the field-and microscale, it could contribute in the development of a new sustainable construction material.

Direct Observations of the Coupling between Quartz Dissolution and Mg-Silicate Formation

“…The dissolution rate of quartz has been established with extensive laboratory experiments − and rate equations are proposed based on those results. , Hence, it is well-known that with an increase in pH the dissolution rate of quartz increases rapidly, especially when it rises above pH 10. Moreover, the presence of alkaline cations is known to enhance the dissolution rate of quartz. , The experimentally obtained data on quartz dissolution do, however, not correspond to the recently described abnormally fast dissolution of quartz within natural rocks that have been weathered at high pH and Mg-rich conditions at the surface . Controversially, that means that the rates obtained from the field are faster than the rates obtained from laboratory experiments.…”

“…In nature, limited fluid flow is proposed to slow down the weathering process through the formation of a surface layer that inhibits further surface reaction, , and is therefore likely to be involved in the discrepancy between dissolution rates obtained from experiments and from field samples. Nanoscale analyses have also identified an amorphous layer around weathered quartz in field samples, that might act as a precursor for a secondary silicate phase . This secondary transformation possibly results in the removal of the amorphous layer hence enabling further dissolution of the underlying quartz, which may explain a higher field dissolution rate despite the initial formation of an amorphous layer.…”

“…High-pH, Mg-rich solutions occur in nature at ultramafic massifs due to the weathering of serpentinite which is typically associated with the dissolution of brucite, Mg­(OH) 2 . , In the specific example of the Feragen Ultramafic Body in SE Norway, quartz is present within glacial deposits that partly cover the serpentinized ultramafic rock, leading to the replacement of the quartz by a nanocrystalline hydrous Mg-silicate phase . This resulting phase formed from a reaction between dissolved brucite and quartz, has cementing properties and is in fact similar to man-made magnesium silicate hydrate cement, also known as M-S-H cement.…”

Growth of Self‐Assembling Tubular Structures of Magnesium Oxy/Hydroxide and Silicate Related With Seafloor Hydrothermal Systems Driven by Serpentinization

“…The precipitate morphology, chemical structure, and crystal or mineral structures seen in our laboratory experiments are thus of interest to compare with those found in submarine alkaline hydrothermal vents. Recently, the formation of natural magnesium silicate hydrate cement has been described from the weathering of serpentinized rocks leading to a reaction of dissolved brucite with quartz (de Ruiter & Austrheim, 2018). Beyond Earth, it is speculated that these formations may be produced in other planets in the presence of silicates and water (Russell et al, 2014).…”

Immobilization of (Aqueous) Cations in Low pH M-S-H Cement

“…The existence of natural magnesium silicate hydrates formed through dissolution and reprecipitation processes of ultramafic rocks suggests the sufficient durability needed for long-term encapsulation [32,33]. M-S-H is also formed upon perturbation of low-pH cement in contact with clayey environment [34].…”