Quartz has been replaced by magnesium silicate hydrate cement at the Feragen ultramafic body in south-east Norway.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.

中文翻译：

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与水合硅酸镁水泥沉淀有关的石英溶解

在挪威东南部的Feragen超镁铁质岩体中，石英已被水合硅酸镁水泥取代。这发生在变形和重结晶的石英颗粒中，这些石英颗粒沉积为冰川，直到覆盖了超镁铁矿的一部分。暴露于超镁铁质岩体的地方，风化会导致高pH（〜 10），富含镁的流体。石英的溶解速率比在当前条件下实验得出的速率方程式建议的要高约3个数量级。石英溶解和水泥沉淀始于晶间晶界，这些晶界作为通过重结晶石英的流体通道。由于在位错位置优先溶解，在石英表面上也广泛存在腐蚀坑。透射电子显微镜显示风化石英颗粒周围的非晶硅层厚度为100-200 nm。我们建议无定形二氧化硅是界面耦合的溶解-沉淀产物，并且无定形二氧化硅随后会与富含Mg的高pH散装流体发生反应，从而沉淀出硅酸镁水合水泥，允许进一步溶解石英，并局部用水泥完全替代石英。该水泥与硅酸镁水合水泥（MSH）的天然等效物，目前对于核废料封装和环保型建筑水泥很感兴趣，但尚未开发出用于商业用途的水泥。这项研究提供了可能有助于MSH水泥进一步发展的新见解。