Earth crust composition

Earth crust composition

More than 90% on the crust is composed of silicate minerals. Most abundant silicates are feldspars (plagioclase (39%) and alkali feldspar (12%)). Other common silicate minerals are quartz (12%) pyroxenes (11%), amphiboles (5%), micas (5%), and clay minerals (5%). The rest of the silicate family comprises 3% of the crust. Only 8% of the crust is composed of non-silicates — carbonates, oxides, sulfides, etc.
Clay minerals are too small to be shown individually. Even with a light microscope you will see only mud or dust depending on whether these minerals are wet or dry. Clay minerals are silicates that are the products of weathering of other silicate minerals, mostly feldspars.
The most abundant rock types in the crust
Rocks are divided into three broad groups: igneous, metamorphic, and sedimentary rocks. The oceanic crust is largely composed of basaltic igneous rocks which are covered by a thin veneer of sediments which are thickest near the margins of the continental landmasses. The continental crust is much thicker and older. The continental crust is also much more variable and structurally very complex. Virtually all the rock types known to man occur in the continental crust. Even meteorites, xenoliths from the mantle, and ophiolites (fragment of former oceanic crust) are constituents of the continental crust because that’s where we found them.
Roughly three fourths of the continental crust is covered by sedimentary rocks and almost all of it is covered by loose sediments (soil, sand, dirt, etc.). We are most likely to encounter these materials, but it is important to understand that despite being so ubiquitous on the surface, they make up only about 8% of the whole mass of the crust. Sediments consolidate to sedimentary rocks after burial. Sand turns to sandstone, limy mud to limestone, clay to claystone. Sedimentary rocks are stable only in the upper parts of the crust. High pressure and temperature in the deeper parts metamorphoses them (minerals recrystallize) to various metamorphic rocks. The bulk of the continental crust is made of metamorphic rocks. Igneous rocks are also common on the surface in volcanically active regions, but they also occur deeper in the crust as granitic (mostly) intrusions.
Important sediments are sand, clay, mud (wet mixture of clay and fine sand), and limy mud. Widespread sedimentary rocks are limestone (2% of the crust by volume), sandstone (1.7%), claystone (4.2%) which are lithified versions of the loose sediments mentioned before. Chemical sediments like halite and gypsum are important as well, but their overall volume is clearly less than 1% of the crust. Important igneous rocks are granite, granodiorite, gabbro, basalt, diorite, andesite, etc. It is very difficult to say what is the percentage of these rocks. Important metamorphic rocks are metamorphosed equivalents of widespread sedimentary and igneous rocks. Common metamorphic rocks are slate (metamorphosed claystone), schist (met. claystone, higher grade than slate) quartzite (met. sandstone), marble (met. limestone), gneiss (met. igneous rock or sedimentary rocks), amphibolite (met. basaltic rocks).
Gypsum is an evaporite mineral. Evaporites are water-soluble chemical sediments that crystallize out of concentrated (high salinity) seawater in lagoons.