The 14th International Conference on

Miniaturized Systems for Chemistry and Life Sciences

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Goldschmidt classification





 

Goldschmidt classification

The Goldschmidt classification, developed by Victor Goldschmidt (1888-1947), is a geochemical classification which groups the chemical elements within the Earth according to their preferred host phases into lithophile (rock-loving), siderophile (iron-loving), chalcophile (ore-loving or chalcogen-loving), and atmophile (gas-loving) or volatile (the element, or a compound in which it occurs, is liquid or gaseous at ambient surface conditions).

The trace radioactive elements (namely Tc, Pm, Po, At, Rn, Fr, Ra, Ac, Pa, Np, and Pu) are shown as synthetic, because their occurrence in nature is fleeting and is entirely dependent on their long-lived parents Th and U, and they are not very mobile. For instance, polonium's chemistry would predict it to be a chalcophile, but in actuality it tends to occur instead as a lithophile along with its parent uranium; even radon, which is a gas, does not usually have time to travel very far from the original uranium source before decaying. When needed, these elements are typically produced synthetically in nuclear reactors instead of using the tedious and laborious process of extraction from uranium ores.

Lithophile elements are those that remain on or close to the surface because they combine readily with oxygen, forming compounds that do not sink into the core. The lithophile elements include: Al, B, Ba, Be, Br, Ca, Cl, Cr, Cs, F, I, Hf, K, Li, Mg, Na, Nb, O, P, Rb, Sc, Si, Sr, Ta, Th, Ti, U, V, Y, Zr, W and the lanthanides.

Most lithophile elements form very stable ions with an electron configuration of a noble gas (sometimes with additional f-electrons). The few that do not, such as silicon, phosphorus and boron, form extremely strong covalent bonds with oxygen – often involving pi bonding. Their strong affinity for oxygen causes lithophile elements to associate very strongly with silica, forming relatively low-density minerals that thus float to the crust. The more soluble minerals formed by the alkali metals tend to concentrate in seawater or extremely arid regions where they can crystallise. The less soluble lithophile elements are concentrated on ancient continental shields where all soluble minerals have been weathered.

Because of their strong affinity for oxygen, most lithophile elements are enriched in the Earth's crust relative to their abundance in the solar system. The most reactive s- and f-block metals, which form either saline or metallic hydrides, are known to be extraordinarily enriched on Earth as a whole relative to their solar abundances. This is because during the earliest stages of the Earth's formation the reaction that controlled the stable form of each chemical element was its ability to form compounds with hydrogen. Under these conditions, the s- and f-block metals were strongly enriched during the formation of the Earth. The most enriched elements are rubidium, strontium and barium, which between them account for over 50 percent by mass of all elements heavier than iron in the Earth's crust.

Owing to their strong affinity for oxygen, lithophile metals, although they form the great bulk of the metallic elements in Earth's crust, were never available as free metals before the development of electrolysis. With this development, many lithophile metals are of considerable value as structural metals (magnesium, aluminium, titanium, vanadium) or as reducing agents (sodium, magnesium, calcium). The process of smelting these metals is extremely energy-intensive. With emissions of greenhouse gases suspected of contributing to climate change, the use of these elements as industrial metals is called into question, despite the depletion of rarer and less reactive chalcophile metals leaving few substitutes.

The non-metals phosphorus and the halogens were also not known to early chemists, though production of these elements is less difficult than of metallic lithophiles since electrolysis is required only with fluorine. Elemental chlorine is particularly important as an oxidizing agent – usually being made by electrolysis of sodium chloride.

Most siderophile elements have practically no affinity whatsoever for oxygen: indeed oxides of gold are thermodynamically unstable with respect to the elements. They form stronger bonds with carbon or sulfur, but even these are not strong enough to separate out with the chalcophile elements. Thus, siderophile elements are bound through metallic bonds with iron in the dense layer of the Earth's core, where pressures may be high enough to keep the iron solid. Manganese, iron, and molybdenum do form strong bonds with oxygen, but in the free state (as they existed on the primitive Earth when free oxygen did not exist) can mix so easily with iron that they do not concentrate in the siliceous crust, as do true lithophile elements. However, ores of manganese are found in much the same sites as are those of aluminium and titanium, owing to manganese's great reactivity towards oxygen.

Because they are so concentrated in the dense core, siderophile elements are known for their rarity in the Earth's crust. Most of them have always been known as precious metals because of this. Iridium is the rarest transition metal occurring within the Earth's crust, with an abundance by mass of less than one part per billion. Mineable deposits of precious metals usually form as a result of the erosion of ultramafic rocks, but are not highly concentrated even compared to their crustal abundances, which are typically several orders of magnitude below their solar abundances. However, because they are concentrated in the mantle and core, siderophile elements are believed to be present in the Earth as a whole (including the core) in something approaching their solar abundances.

The most metallic chalcophile elements (of the copper, zinc and boron groups) may mix to some degree with iron in the Earth's core. They are not likely to be depleted on Earth as a whole relative to their solar abundances since they do not form volatile hydrides. Zinc and gallium are somewhat "lithophile" in nature because they often occur in silicate or related minerals and form quite strong bonds with oxygen. Gallium, notably, is sourced mainly from bauxite, an aluminum hydroxide ore in which gallium ion substitutes for chemically similar aluminum.







 

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