Free SiO-2 is represented by quartz, coesite and stishovite polymorphs. Quartz widely occurs in different rocks of the earth's crust, whereas coesite and stishovite are only formed under high pressures and temperatures. Since olivine is silica-unsaturated, quartz and olivine never coexist in the same rock. Therefore, in mantle peridotite and other ultramafic rocks, primary quartz can never be found. Recently, free SiO-2 (quartz and coesite) was discovered from podiform chromitites in mantle peridotites of Luobusa ophiolite, Tibet. High-pressure and high-temperature phase equilibrium experiments show that silicates such as olivine and pyroxene will be decomposed into simple oxides like FeO (wustite), MgO (periclase) and SiO-2 (stishovite) beneath the lower mantle (>670 km). It is thus thought that the free silica (quartz and coesite) associated with mantle minerals was possibly derived from the lower mantle and that mantle plume might have been the mechanism for transporting these minerals to the shallow part of the ophiolitic mantle.