Many of the important minerals in the lithosphere are silicates. Most silicates consist of polymeric silicon-oxygen anions and metal cations. In silicates each silicon is surrounded by four oxygens. Various numbers of oxygens are shared with adjacent silicon atoms. number of oxygens shared silicate type example two single chain two and three double chain asbestos three sheet clay Sharing of all four oxygens results in a neutral three dimensional structure of formula SiO₂. One form of this is quartz. In aluminosilicates some Si(IV)'s in the anion are replaced by Al(III). As the overall charge on the anion more negative and extra cations are found in these structures. Mica is a sheet aluminosilicate. The zeolites used as catalysts in the Mobil process are aluminosilicates with all four oxygens shared.