silicate, known as a tectosilicate, each tetrahedron shares all 4 oxygen atoms with its neighbours, forming a 3D structure. Quartz and feldspars are in this group. Silicates with non-tetrahedral silicon Although the tetrahedron is a common coordination geometry for silicon(IV) compounds, silicon may also occur with higher coordination numbers. For example, in the anion hexafluorosilicate , the silicon atom is surrounded by six fluorine atoms in an octahedral arrangement. This structure is also seen in the hexahydroxysilicate anion that occurs in thaumasite, a mineral found rarely in nature but sometimes observed among other calcium silicate hydrates artificially formed in cement and concrete submitted to a severe sulfate attack. At very high pressure, such as exists in the majority of the earth's crust, even SiO2 adopts the six-coordinated octahedral geometry in the mineral stishovite, a dense polymorph of silica found in the lower mantle of the Earth and also formed by shock during meteorite impacts. Chemical properties Silicates with alkali cations and small or chain-like anions, such as sodium ortho- and metasilicate, are fairly soluble in water. They form several solid hydrates when crystallized from solution. Soluble sodium silicates and mixtures thereof, known as waterglass are in fact important industrial and household chemicals. Silicates of non-alkali cations, or with sheet and tridimensional polymeric anions, generally have negligible solubility in water at normal conditions. Reactions Silicates are generally inert chemically. Hence they are common minerals. Their resiliency also recommends their use as building materials. When treated with calcium oxides and water, silicate minerals form Portland cement. Equilibria involving hydrolysis of silicate minerals are difficult to study. The chief challenge is the very low solubility of SiO44- and its various protonated forms. Such equilibria are relevant to the processes occurring on geological time scales. Some plants excrete ligands that dissolve silicates, a step in biomineralization. Detection Silicate anions in solution react with molybdate anions yielding yellow silicomolybdate complexes. In a typical preparation, monomeric orthosilicate was found to react completely in 75 seconds; dimeric pyrosilicate in 10 minutes; and higher oligomers in considerably longer time. In particular, the reaction is not observed with suspensions of colloidal silica. Zeolite formation The nature of soluble silicates is relevant to understanding biomineralization and the synthesis of aluminosilicates, such as the industrially important catalysts called zeolites. See also Alkali-silica reaction Carbon cycle Carbonate-silicate cycle Ocean acidification References Silicates