Skip to main content
Select Source:

Geochemistry

Geochemistry

Geochemistry is the study of the chemical processes that form and shape the earth.

Earth is essentially a large mass of crystalline solids that are constantly subject to physical and chemical interaction with a variety of solutions (e.g., water ) and substances. These interactions allow a multitude of chemical reactions.

It is through geochemical analysis that estimates of the age of Earth are formed. Because radioactive isotopes decay at measurable and constant rates (e.g., half-life ) that are proportional to the number of radioactive atoms remaining in the sample, analysis of rocks and minerals can also provide reasonably accurate determinations of the age of the formations in which they are found. The best measurements obtained via radiometric dating (based on the principles of nuclear reactions) estimate the age of Earth to be four and one half billion years old.

Dating techniques combined with spectroscopic analysis provide clues to unravel Earth's history. Using neutron activation analysis, Nobel Laureate Luis Alvarez discovered the presence of the element iridium when studying samples from the K-T boundary layer (i.e., the layer of sediment laid down at the end of the Cretaceous and beginning of the Tertiary Periods). Fossil evidence shows a mass extinction at the end of the Cretaceous Period , including the extinction of the dinosaurs. The uniform iridium layerand presence of quartz crystals with shock damage usually associated only with large asteroid impacts or nuclear explosionsadvanced the hypothesis that a large asteroid impact caused catastrophic climatic damage that spelled doom for the dinosaurs.

Although hydrogen and helium comprise 99.9% of the atoms in the universe, Earth's gravity is such that these elements readily escape Earth's atmosphere. As a result, the hydrogen found on Earth is found bound to other atoms in molecules.

Geochemistry generally concerns the study of the distribution and cycling of elements in the crust of the earth. Just as the biochemistry of life is centered on the properties and reaction of carbon , the geochemistry of Earth's crust is centered upon silicon . Also important to geochemistry is oxygen . Oxygen is the most abundant element on Earth. Together, oxygen and silicon account for 74% of Earth's crust.

The type of magma (Basaltic, Andesitic or Ryolytic) extruded by volcanoes and fissures (magma is termed lava when at Earth's surface) depends on the percentage of silicon and oxygen present. As the percentage increases, the magma becomes thicker, traps more gas, and is associated with more explosive eruptions.

The eight most common elements found on Earth, by weight, are oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg).

Unlike carbon and biochemical processes where the covalent bond is most common, however, the ionic bond is the most common bond in geology . Accordingly, silicon generally becomes a cation and will donate four electrons to achieve a noble gas configuration. In quartz, each silicon atom is coordinated to four oxygen atoms. Quartz crystals are silicon atoms surrounded by a tetrahedron of oxygen atoms linked at shared corners.

Rocks are aggregates of minerals and minerals are composed of elements. A mineral has a definite (not unique) formula or composition. Diamonds and graphite are minerals that are polymorphs (many forms) of carbon. Although they are both composed only of carbon, diamonds and graphite have very different structures and properties. The types of bonds in minerals can affect the properties and characteristics of minerals.

Pressure and temperature affect the structure of minerals. Temperature can determine which ions can form or remain stable enough to enter into chemical reactions. Olivine , ((Fe, Mg)2 SiO4), for example is the only solid that will form at 1,800°C. According to olivine's formula, it must be composed of two atoms of either Fe or Mg. Olivine is built by the ionic substitution of Fe and Mgthe atoms are interchangeable because they the same electrical charge and are of similar sizeand thus, olivine exists as a range of elemental compositions termed a solid solution series . Olivine can thus be said to be "rich" in iron or rich in magnesium. As magma cools larger atoms such as potassium ions enter into reactions and additional minerals form.

The determination of the chemical composition of rocks involves the crushing and breakdown of rocks until they are in small enough pieces that decomposition by hot acids (hydrofluoric, nitric, hydrochloric, and perchloric acids) allows the elements present to enter into solution for analysis. Other techniques involve the high temperature fusion of powdered inorganic reagent (flux) and the rock . After melting the sample, techniques such as x-ray fluorescence spectrometry may be used to determine which elements are present.

Chemical and mechanical weathering break down rock through natural processes. Chemical weathering of rock requires water and air. The basic chemical reactions in the weathering process include solution (disrupted ionic bonds), hydration, hydrolysis, and oxidation.

The geochemistry involved in many environmental issues has become an increasing important aspect of scientific and political debate. The effects of acid rain are of great concern to geologists not only for the potential damage to the biosphere , but also because acid rain accelerates the weathering process. Rainwater is made acidic as it passes through the atmosphere. Although rain becomes naturally acidic as it contacts nitrogen, oxygen, and carbon dioxide in the atmosphere, many industrial pollutants bring about reactions that bring the acidity of rainwater to dangerous levels. Increased levels of carbon dioxide from industrial pollution can increase the formation of carbonic acid. The rain also becomes more acidic. Precipitation of this "acid rain" adversely affects both geological and biological systems.

According to plate tectonic theory, the crust (lithosphere ) of Earth is divided into shifting plates. Geochemical analysis of Earth's tectonic plates reveals a continental crust that is older, thicker and more granite-like than the younger, thinner oceanic crusts made of basaltic (iron, magnesium) materials.

See also Atomic mass and weight; Atomic number; Atomic structure; Big Bang theory; Chemical bonds and physical properties; Chemical elements; Geologic time

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Geochemistry." World of Earth Science. . Encyclopedia.com. 23 Jun. 2017 <http://www.encyclopedia.com>.

"Geochemistry." World of Earth Science. . Encyclopedia.com. (June 23, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/geochemistry

"Geochemistry." World of Earth Science. . Retrieved June 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/geochemistry

Chalcophiles, Lithophiles, Siderophiles, and Atmophiles

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 (18881947) 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 affinityusually in the form of covalent bondswith 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 structureespecially the number of electrons available for bondingelement 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.

See also Chemical bonds and physical properties; Chemical elements

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Chalcophiles, Lithophiles, Siderophiles, and Atmophiles." World of Earth Science. . Encyclopedia.com. 23 Jun. 2017 <http://www.encyclopedia.com>.

"Chalcophiles, Lithophiles, Siderophiles, and Atmophiles." World of Earth Science. . Encyclopedia.com. (June 23, 2017). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/chalcophiles-lithophiles-siderophiles-and-atmophiles

"Chalcophiles, Lithophiles, Siderophiles, and Atmophiles." World of Earth Science. . Retrieved June 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/chalcophiles-lithophiles-siderophiles-and-atmophiles

geochemistry

geochemistry, study of the chemical changes on the earth. More specifically, it is the study of the absolute and relative abundances of chemical elements in the minerals, soils, ores, rocks, water, and atmosphere of the earth and the distribution and movement of these elements from one place to another as a result of their chemical and physical properties. Geochemical studies also include the study of isotopes of chemical elements, especially their abundance and stability in the universe. Geochemistry provides a theoretical basis for ore prospecting and has refined and improved the methods of determining the age of rocks including the use of radioactive isotopes to date the rock. Chemical studies of ancient sedimentary rocks and the fluids contained in them have provided insights into the evolution of the oceans and the atmosphere. Experiments have been conducted with gases that recreate the primordial atmosphere. Today, important work in geochemistry involves the study of geochemical cycles in the atmosphere; marine and estuarine waters; and the earth's crust. There are many studies in relation to the effects of massive amounts of pollutants on the environment.

See K. B. Krauskopf, Introduction to Geochemistry (1967); G. Faure, Principles and Applications of Geochemistry (1991).

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"geochemistry." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 23 Jun. 2017 <http://www.encyclopedia.com>.

"geochemistry." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (June 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/geochemistry

"geochemistry." The Columbia Encyclopedia, 6th ed.. . Retrieved June 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/geochemistry

geochemistry

geochemistry Branch of geology concerned with the abundance and distribution of the chemical elements and their isotopes within the Earth or within solid bodies in the solar system, their circulation in natural systems (the atmosphere, hydrosphere, biosphere, and lithosphere), and the laws governing this distribution and its evolution.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"geochemistry." A Dictionary of Earth Sciences. . Encyclopedia.com. 23 Jun. 2017 <http://www.encyclopedia.com>.

"geochemistry." A Dictionary of Earth Sciences. . Encyclopedia.com. (June 23, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/geochemistry

"geochemistry." A Dictionary of Earth Sciences. . Retrieved June 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/geochemistry

geochemistry

ge·o·chem·is·try / ˌjēōˈkeməstrē/ • n. the study of the chemical composition of the earth and its rocks and minerals. DERIVATIVES: ge·o·chem·i·cal / -ˈkemikəl/ adj. ge·o·chem·ist / -ˈkemist/ n.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"geochemistry." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 23 Jun. 2017 <http://www.encyclopedia.com>.

"geochemistry." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (June 23, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/geochemistry

"geochemistry." The Oxford Pocket Dictionary of Current English. . Retrieved June 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/geochemistry

geochemistry

geochemistry Study of the chemical composition of Earth. It is concerned with the abundance, distribution and migration of the elements in Earth's atmosphere, mantle, crust and core.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"geochemistry." World Encyclopedia. . Encyclopedia.com. 23 Jun. 2017 <http://www.encyclopedia.com>.

"geochemistry." World Encyclopedia. . Encyclopedia.com. (June 23, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/geochemistry

"geochemistry." World Encyclopedia. . Retrieved June 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/geochemistry