Understanding Silica from the Viewpoint of Asymmetry.

Silica is abundant in the Earth's crust, and silicate materials are used on the global scale, from industrial products for architecture, vehicles, electronics, and optics to consumption as foods, medicines, supplements, and cosmetics. Silica has become increasingly important in current science and technology, as seen in the components of advanced materials. The characteristic formula of silica is very simply often expressed as SiO2 . However, it is difficult to draw silica precisely owing to its intricate chemical structures. We need to make a greater effort in understanding silica, even though silica chemistry has existed for over 200 years. Similar to the homochirality observed in natural amino acids, natural silica of quartz is chiral, and in some sense, the origin of life with chirality might be partly related to quartz-like silica chirality. This review focuses on the asymmetry of silica from the view of the formation of irregular tetrahedron structures of SiO4 . Silica is composed of several repeated tetrahedron units of SiO4 , leading to the formation of inorganic polymers with divergently expanded 3D structures. In this large polymeric skeleton, not every unit of SiO4 can maintain an ideal tetrahedron, and thus, it becomes twisted. The twisting results in an asymmetric point on the Si atom, leading silica to become racemic in the stereochemical sense. Therefore, enantioselective preparation of silica should endow silica with chirality through the silica skeleton. Some recent achievements exhibit that silica is an effective chiral material and has great potential for transferring chirality from silica to other matter.