Chalcophiles, Lithophiles, Siderophiles, and Atmophiles
Chalcophiles, lithophiles, siderophiles, and atmophiles are classes of elements based upon similar geochemical properties and reactive affinities. The classes were originally advanced by Swiss-born Victor Goldschmidt (1888–1947) and are terms still widely used by geologists and geochemists. The key factor in determining an element's class is the type of chemical bonds that the element forms.
Chalcophile elements have a high bonding affinity—usually in the form of covalent bonds—with sulfur, and are, accordingly, usually abundant in sulfides. Chalcophiles also exhibit a bonding affinity with selenium, tellurium, arsenic, and antimony and therefore also exhibit high levels of derivatives of these elements. When sulfur is abundant, chalcophile elements readily form sulfide minerals as they precipitate from the magma. This process partially explains the formation of extensive deposits of iron-nickel-copper sulfides.
Lithophiles have a high bonding affinity with oxygen. Lithophiles have an affinity to form ionic bonds and are represented by silicates (silicon and oxygen) in the crust and mantle. Other lithophile elements include magnesium, aluminum, sodium, potassium, iron, and calcium.
Siderophiles exhibit a weak affinity to both oxygen and sulphur. Siderophiles have an affinity for iron and a distinguishing characteristic of siderophiles is that they exhibit high solubility in molten iron. Siderophile elements generally have a low reactivity and exhibit an affinity to form metallic bonds. As a result, siderophiles are most often found in their native state. Not abundant in the core or mantle, most siderophiles are thought to be richest at Earth's core. Platinum (Pt) group metals, including Ruthium (Ru), Rhodium (Rd), Palladium (Pd), Osmium (Os), and Iridium (Ir), show exhibit a strong siderophile tendency.
Atmophiles are a related fourth class of elements characterized by their ability to form van der Waals bonds. Atmophiles are also highly volatile.
Chalcophiles, lithophiles, siderophiles, and atmophiles have differing densities. Accordingly, after formation from the molten state, these differential densities tend to separate the classes. For example, siderophiles have a greater average density than lithophiles and thus lithophiles would tend to "rise" in the molten state relative to siderophiles. Although the element classes were derived, in part, from an attempt to explain the distribution of elements, the density differences do not always result in the expected distribution of classes in the earth's core, mantle and crust.
Because geochemical reactivity is a function of electron structure—especially the number of electrons available for bonding—element classes tend to follow groupings or trend as related to the periodic table. The difference in the classification of elements can also be linked to differing valence states.
It is possible for some elements to be assigned to more than one group. The reactivity of an element can also be driven by the relative amounts of elements surrounding it. For example, iron, when in an oxygen deprived environment (e.g., at the earth's core) acts as a siderophile. In the more oxygen rich environment of the crust and mantle, iron acts as a lithophile or chalcophile, and in this form is commonly found in igneous rocks. When sulfur is present is found in sulfide deposits. Siderophiles when surrounded by sulfur, arsenic, and antimony may act as chalcophiles.
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