Stishovite has the same chemical formula as silica quartz (SiO2) but it has much higher density. While SiO2 is abundant on Earth’s crust, it is also a major component of basalt, a type of igneous rock that is rich in iron and magnesium. When basalt in the oceanic crust subducts into Earth’s deep interiors, it is subjected to high pressures and temperatures, triggering basaltic melting and fluid release, i.e., liquid water. These fluids can then react with the surrounding mantle rock, changing its composition.
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Source: Phys.org