following the formula , similar to the behaviour of boron, aluminium, and iron, among other elements. Hence, although some simple silicic acids have been identified in dilute solutions, such as orthosilicic acid and metasilicic acid , none of these are likely to exist in the solid state. Silicate minerals About 95% of the Earth's crustal rocks are made of silica or silicate and aluminosilicate minerals, as reflected in oxygen, silicon, and aluminium being the three most common elements in the crust (in that order). Measured by mass, silicon makes up 27.7% of the Earth's crust. Pure silicon crystals are very rarely found in nature, but notable exceptions are crystals as large as to 0.3 mm across found during sampling gases from the Kudriavy volcano on Iturup, one of the Kuril Islands. Silicate and aluminosilicate minerals have many different structures and varying stoichiometry, but they may be classified following some general principles. Tetrahedral SiO|4 units are common to almost all these compounds, either as discrete structures, or combined into larger units by the sharing of corner oxygen atoms. These may be divided into neso-silicates (discrete SiO|4 units) sharing no oxygen atoms, soro-silicates (discrete Si|2|O|7 units) sharing one, cyclo-silicates (closed ring structures) and ino-silicates (continuous chain or ribbon structures) both sharing two, phyllo-silicates (continuous sheets) sharing three, and tecto-silicates (continuous three-dimensional frameworks) sharing four. The lattice of oxygen atoms that results is usually close-packed, or close to it, with the charge being balanced by other cations in various different polyhedral sites according to size. The orthosilicates MSiO (M = Be, Mg, Mn, Fe, Zn) and are neso-silicates. (phenacite) is unusual as both BeII and SiIV occupy tetrahedral four-coordinated sites; the other divalent cations instead occupy six-coordinated octahedral sites and often isomorphously replace each other as in olivine, . Zircon, , demands eight-coordination of the ZrIV cations due to stoichiometry and because of their larger ionic radius (84 pm). Also significant are the garnets, [MM(SiO)], in which the divalent cations (e.g. Ca, Mg, Fe) are eight-coordinated and the trivalent ones are six-coordinated (e.g. Al, Cr, Fe). Regular coordination is not always present: for example, it is not found in , which mixes six- and eight-coordinate sites for CaII. Soro-silicates, involving discrete double or triple tetrahedral units, are quite rare: metasilicates involving cyclic "2n−" units of corner-abutting tetrahedra forming a polygonal ring are also known. Chain metasilicates, {SiO}, form by corner-sharing of an indefinite