The metastable paragenesis of corundum and quartz is rare in nature but common in laboratory experiments where according to thermodynamic predictions aluminum–silicate polymorphs should form. We demonstrate here that the existence of a hydrous, silicon-bearing, nanometer-thick layer (called “HSNL”) on the corundum surface can explain this metastability in experimental studies without invoking unspecific kinetic inhibition. We investigated experimentally formed corundum reaction products synthesized during hydrothermal and piston–cylinder experiments at 500–800 °C and 0.25–1.8 GPa and found that this HSNL formed inside and on the corundum crystals, thereby controlling the growth behavior of its host. The HSNL represents a substitution of Al with Si and H along the basal plane of corundum. Along the interface of corundum and quartz, the HSNL effectively isolates the bulk phases corundum and quartz from each other, thus apparently preventing their reaction to the stable aluminum silicate. High temperatures and prolonged experimental duration lead to recrystallization of corundum including the HSNL and to the formation of quartz + fluid inclusions inside the host crystal. This process reduces the phase boundary area between the bulk phases, thereby providing further opportunity to expand their coexistence. In addition to its small size, its transient nature makes it difficult to detect the HSNL in experiments and even more so in natural samples. Our findings emphasize the potential impact of nanometer-sized phases on geochemical reaction pathways and kinetics under metamorphic conditions in one of the most important chemical systems of the Earth’s crust.

刚玉和石英的亚稳态共生在自然界中很少见，但在实验室实验中很普遍，根据热力学预测，应形成铝硅酸盐多晶型物。我们在这里证明，刚玉表面上存在含水的，含硅的，纳米厚的层（称为“ HSNL”），可以在实验研究中解释这种亚稳性，而无需进行非特异性的动力学抑制。我们研究了在500–800°C和0.25–1.8 GPa的水热和活塞缸实验期间合成的实验形成的刚玉反应产物，发现该HSNL在刚玉晶体内部和上方形成，从而控制了其主体的生长行为。HSNL代表沿刚玉的基面用Si和H取代Al。沿着刚玉和石英的界面，HSNL有效地将刚玉相和石英相与石英相隔离，因此显然阻止了它们与稳定的硅酸铝的反应。高温和延长的实验持续时间会导致刚玉（包括HSNL）重结晶，并在主体晶体内部形成石英+流体包裹体。该过程减小了本体相之间的相边界面积，从而提供了进一步的机会来扩展它们的共存。除了体积小以外，它的瞬态特性使得在实验中甚至在天然样品中都很难检测到HSNL。我们的发现强调了变质条件下地壳最重要的化学系统之一中纳米级相对地球化学反应途径和动力学的潜在影响。