Metamorphosis is the only major process in which quartz is either produced or consumed and disappears from the environment during the formation of new minerals.įinally, quartz and all the other silica polymorphs are absent from the Earth's upper mantle rocks, and the presence of silicon dioxide polymorphs stishovite and seifertite that are stable under these conditions in the lower mantle is uncertain. The total amount of quartz present in metamorphic rocks is lower than in igneous but higher than in sedimentary rocks. Very high concentrations of quartz are found quartzite and certain schists, while rocks derived from limestones and certain igneous rocks may be void of it. The quartz content often reflects the mineral composition of the precursor rock. Metamorphic rocks also show large variations in quartz content. The abundance of sandstones and sands at the Earth's surface is a bit misleading, as sedimentary rocks cover most of the Earth, but only make up a small part of the Earth's crust volume, so the total amount of quartz in them is rather low. On the other hand, limestones are also very common and typically have a very low quartz content. Very high concentrations of quartz can be found in certain sedimentary rocks like sandstone, as well as in alluvial and marine sands and sand dunes. On the other hand, the continental crust contains large amounts of granitic rocks that are rich in quartz, so almost all quartz is found in the continental crust that covers just about 40% of the Earth's surface. These rocks have a very low quartz content or are void of it. When one talks about the Earth's crust as a whole, volcanic rocks are more common at the surface than granitoids, because that encompasses the oceanic crust that is largely made of basalt and gabbro. The largest amount of quartz is contained in igneous rocks, in particular in so-called granitoids, granites and related rocks. Metamorphic rocks are estimated to contribute 27.4% to the volume of the Earth's crust. This is a very diverse class of rocks with a wide range of mineral compositions.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |