Elements in the Earth’s crust

The Earth can be divided into three basic layers. The first and most internal one is called the core. It is followed by the mantle, which forms the middle layer, with its upper section classified to the lithosphere (the external shell of the Earth). The last and most external layer is the crust. It is 10 to 70 kilometres deep and represents only 1.4% of the planet’s volume and 0.3% of its mass. Even so, it is physically and chemically the most varied geosphere. The combination of geographical and chemical sciences gave rise to the geology, which is a field of science that deals with the structure, properties and history of planets. Its development enabled us to determine such things as the elemental composition of the Earth's crust.

Published: 31-03-2023

Shaping the Earth

Resulting from the continuous movement of tectonic plates, rock-forming processes may last millions of years. Due to various displacements of rocks which occurred in history, many layers were removed from the Earth’s surface into its interior to be replaced by rocks formed later or just the opposite, they were pushed to the surface. This is why geologists can examine not only the rocks composing the surface layer but also those which were earlier formed and located even a dozen or so kilometres inside the Earth. As the deeper layers are unreachable due to the conditions inside them, i.e. high pressures and temperatures (even 6,000°C inside the core), all information about them was acquired with indirect methods, which use such phenomena as seismic waves, earthquakes or volcano eruptions. On the contrary, since the geosphere up to around 16 km is relatively easy to access, we can determine the average chemical composition of the lithosphere and so also the Earth’s crust.

The crust

That most external layer of the Earth is also the one that is the most diverse and most known to the man. It is a geosphere which has the lowest temperature, thickness and mass. From the inside, it borders the Earth’s mantle at the Mohorovičić discontinuity, while from the outside it directly contacts the atmosphere or hydrosphere. The crust’s chemical composition includes 93 elements, but only 8 of them represent as much as 99.5% of its mass. It is assumed that, in percent by weight, oxygen represents 46.6%, silicon 27.72%, aluminium 8.13%, iron 5.00%, calcium 3.63%, sodium 2.83%, potassium 2.60%, and magnesium 2.08%. For comparison: the content of hydrogen is estimated at 0.14%, sulphur at 0.05%, carbon at 0.03%, and copper at 0.01%. The composition also includes chlorine, rubidium, fluorine, strontium, barium, nickel, lithium, nitrogen and many more. The diversity of that geosphere results from the abundance of many structures, such as igneous, metamorphic, and sedimentary rocks in various configurations. Due to the mentioned geological processes, the formed rocks do not show the same chemical composition, which varies both horizontally and vertically. In addition, it is very uncommon for an element to exist in its native form; elements normally form clusters of different atoms in the form of minerals. A few of them, such as gold, silver, copper, sulphur, diamond, and graphite, contain only one element. Multi-elemental minerals are much more numerous; at present, we know around 3,000 of them, and silicates are regarded as the most common.

Minerals

Minerals are defined as chemical compounds formed as a result of natural processes, which have a specified chemical composition and a crystalline structure. We can classify them into types based on their different chemical and physical properties. There are many types of minerals, as the same chemical compound may exist in various crystalline forms. For example, calcium carbonate forms three minerals with the same chemical properties but different physical properties, namely: chalk, limestone and marble. According to the Nickel-Strunz classification, minerals are divided into nine categories: native elements, sulphides, halides, oxides and hydroxides, nitrates, carbonates and borates, sulphates, chromates, molybdates, tungstates, phosphates, arsenates, vanadates, silicates, and organic compounds. The minerals that contain the most common elements are rock-forming.

Rocks

A rock is a massive aggregate of minerals. The mineral composition of rocks can be diverse. There are one-mineral variants such as calcite limestones or marbles, referred to as monomineralic rocks. However, the vast majority are polymineralic rocks, which are composed of at least two minerals. Common ones include limestone rocks or granite rocks (where quartz is the principal mineral). The most common of them, silicates, represent three fourths of the Earth’s crust. They can be found as rocks, sands, soils and clays, and their composition is dominated by silica (read about silica fume) and silicic acid salts.

Examples of ores

The group containing sulphides also includes sulphosalts, arsenides, antimonides, bismuthides, selenides and tellurides, which are similar to sulphides, e.g. they show similar physical characteristics, including high density and strong metallic shine. Most of them are formed from hydrothermal solutions, but they can also crystallise from sulphide magma. Although they are not very common (0.15% by weight of the Earth’s crust), they are important for the economy as precious metal (gold, silver, platinum) ores. This group of minerals includes, for example, the alabandite MnS, antimonite Sb2S3, arsenopyrite FeAsS, and cinnabar HgS. Halides mainly include the compounds of chlorine and fluorine, rarely bromides and iodides. Light metal halides are characterised by their glassy shine and a low refractive index. They are mainly found in pegmatites and hydrothermal veins. They are used for the production of mineral fertilisers, chemical raw materials, and fluxes; some of them can be directly consumed. Minerals from this group include, for example, halite NaCl, fluorite Ca2F, sylvite KCl, and atacamite Cu2(OH)3Cl. Even though the oxygen is the most common element in the Earth’s crust, its minerals represent only 4.5%. Almost all of them are contained in iron oxide ore, with other compounds being aluminium, magnesium, titanium and chromium oxides. Minerals from this group are formed under magmatic, hydrothermal and weathering conditions. Some of them are raw materials important for the industry, others are valuable jewellery stones. They include chromite FeCr2O4, haematite Fe2O3, ruby, and sapphire Al2O3. Sulphates and chromates constitute a very large group of nearly 200 minerals. They are characterised by their transparency and glassy shine. They are formed at low temperatures and pressures, in oxygen-rich conditions. This group includes gypsum CaSO4·2H2O, anhydrite CaSO4, and celestine SrSO4. In the nature, there are about 70 carbonates, the most important being calcite, which is capable of forming sedimentary, metamorphic and igneous rocks. These represent limestones, marbles and carbonatites, respectively. Normally, they are formed in hydrothermal veins and sedimentation basins. They are very important due to their application in the chemical, metallurgical and cement industries as iron, manganese and zinc ores. Carbonate minerals include aragonite and calcite CaCO3, azurite Cu3(CO3)2(OH)2, dolomite CaMg(CO3)2, and magnesite MgCO3.


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