Geochemistry

Geochemists study the chemical composition of Earth and other planets.

The field of geochemistry involves study of the chemical composition of Earth and extraterrestrial bodies and systems, and the chemical processes and reactions that take place within them. It also involves investigation of the cycles of matter and energy that transport the Earth's chemical constituents through time and space.

Scientific studies in geochemistry provide knowledge about Earth and its history, and they help us understand some of the processes involved in the formation of valuable mineral deposits and in changing the planet's climate. Geochemical knowledge is also useful when making plans to dispose of toxic wastes in a manner that causes least harm to humans and the environment.

Contents

Mineralogists Victor Goldschmidt and Vladimir Vernadsky are generally considered the founders of modern geochemistry. Goldschmidt enunciated many of the ideas in this field in a series of publications (from 1922) under the title Geochemische Verteilungsgesetze der Elemente. Vernadsky's book on geochemistry was published in Russian in 1924.

Subfields

Meteorites may be studied as part of cosmochemistry.

Geochemistry includes the following major subfields and areas of study.

  • Cosmochemistry: It deals with analysis of the distribution of elements and their isotopes in extraterrestrial bodies and systems. Studies in cosmochemistry include attempts to understand the formation of and chemical processes within the Solar System, the origin of meteorites, and the formation of elements in stars.
  • Examination of the distribution and movements of elements in different parts of Earth (the crust, mantle, hydrosphere, and so forth) and in minerals, with the goal of determining the underlying systems of distribution and transport.
  • Isotope geochemistry: It involves determining the distribution and concentrations of the isotopes of elements in terrestrial and extraterrestrial materials. The knowledge gained may be used to determine the age of these materials and the historical changes they have gone through.
  • Organic geochemistry: This area involves studying the role of carbon-containing compounds and processes derived from living or once-living organisms. This area of geochemistry helps us understand how living things affect chemical cycles, and the formation of petroleum, coal, natural gas, and ores.
  • Regional, environmental and exploration geochemistry: It involves studies related to environmental, hydrological, and mineral exploration.

Chemical characteristics of rocks

The more common constituents of rocks on Earth are oxides. The main exceptions to oxides are compounds of chlorine, sulfur, and fluorine.

According to calculations by F. W. Clarke, a little more than 47 percent of Earth's crust consists of oxygen. It occurs mainly in the form of oxides, particularly silica, alumina, iron oxides, lime, magnesia, potash, and soda. Silica functions principally as an acid, forming silicates, and the most common minerals of igneous rocks are silicates. From a computation based on 1,672 analyses of all kinds of rocks, Clarke arrived at the following values for the average percentage composition: SiO2=59.71; Al2O3=15.41; Fe2O3=2.63; FeO=3.52; MgO=4.36; CaO=4.90; Na2O=3.55; K2O=2.80; H2O=1.52; TiO2=0.60; and P2O5=0.22. (The total of these is 99.22 percent). All other constituents occur in very small quantities, usually much less than one percent.

The oxides combine in various ways. Some examples are given below.

  • Potash and soda combine to produce mostly feldspars, but may also produce nepheline, leucite, and muscovite.
  • Phosphoric acid with lime forms apatite.
  • Titanium dioxide with ferrous oxide gives rise to ilmenite.
  • Magnesia and iron oxides with silica crystallize as olivine or enstatite, or with alumina and lime form the complex ferro-magnesian silicates (such as the pyroxenes, amphiboles, and biotites).
  • Any silica in excess of that required to neutralize the bases separates out as quartz; excess alumina crystallizes as corundum.

These combinations must be regarded only as general tendencies, for there are numerous exceptions to the rules. The prevalent physical conditions also play a role in the formation of rocks.

Clarke also calculated the relative abundances of the principal rock-forming minerals and obtained the following results: apatite=0.6 percent, titanium minerals=1.5 percent, quartz=12.0 percent, feldspars=59.5 percent, biotite=3.8 percent, hornblende and pyroxene=16.8 percent, for a total of 94.2 percent. These figures, however, can only be considered rough approximations.

Acid, intermediate, basic, and ultrabasic igneous rocks

Rocks that contain the highest levels of silica and on crystallization yield free quartz are placed in a group generally designated "acid" rocks. Rocks that contain lowest levels of silica and most magnesia and iron, so that quartz is absent while olivine is usually abundant, form the "basic" group. The "intermediate" group includes rocks characterized by the general absence of both quartz and olivine. An important subdivision of these contains a very high percentage of alkalis, especially soda, and consequently has minerals such as nepheline and leucite not common in other rocks. It is often separated from the others as the "alkali" or "soda" rocks, and there is a corresponding series of basic rocks. Lastly, a small group rich in olivine and without feldspar has been called "ultrabasic" rocks. They have very low percentages of silica but high proportions of iron and magnesia.

Except for the last group, practically all rocks contain feldspars or feldspathoid minerals. In acid rocks, the common feldspars are orthoclase, with perthite, microcline, oligoclase, all having much silica. In the basic rocks, labradorite, anorthite, and bytownite prevail, being rich in lime and poor in silica, potash and soda. Augite is the most common ferro-magnesian of the basic rocks, while biotite and hornblende are usually more frequent in acid rocks.

Commonest Minerals Acid Intermediate Basic Ultrabasic
Quartz
Orthoclase (and Oligoclase), Mica, Hornblende, Augite
Little or no Quartz:
Orthoclase hornblende, Augite, Biotite
Little or no Quartz:
Plagioclase Hornblende, Augite, Biotite
No Quartz
Plagioclase Augite, Olivine
No Felspar
Augite, Hornblende, Olivine
Plutonic or Abyssal type Granite Syenite Diorite Gabbro Peridotite
Intrusive or Hypabyssal type Quartz-porphyry Orthoclase-porphyry Porphyrite Dolerite Picrite
Lavas or Effusive type Rhyolite, Obsidian Trachyte Andesite Basalt Limburgite

Rocks that contain leucite or nepheline, either partly or wholly replacing feldspar, are not included in the above table. They are essentially of intermediate or basic character. They may be regarded as varieties of syenite, diorite, gabbro, and so forth, in which feldspathoid minerals occur. Indeed there are many transitions between ordinary syenites and nepheline (or leucite) syenite, and between gabbro or dolerite and theralite or essexite. But because many minerals that develop in these "alkali" rocks are uncommon elsewhere, it is convenient in a purely formal classification like that outlined here to treat the whole assemblage as a distinct series.

Nepheline and Leucite-bearing Rocks
Commonest Minerals Alkali Feldspar, Nepheline or Leucite, Augite, Hornblend, Biotite Soda Lime Feldspar, Nepheline or Leucite, Augite, Hornblende (Olivine) Nepheline or Leucite, Augite, Hornblende, Olivine
Plutonic type Nepheline-syenite, Leucite-syenite, Nepheline-porphyry Essexite and Theralite Ijolite and Missourite
Effusive type or Lavas Phonolite, Leucitophyre Tephrite and Basanite Nepheline-basalt, Leucite-basalt

The above classification is based essentially on the mineralogical constitution of igneous rocks. Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial, but it has developed with the growth of the science and is still adopted as the basis on which smaller subdivisions have been set up.

The subdivisions are by no means of equal value. For example, the syenites and the peridotites are far less important than the granites, diorites, and gabbros. Moreover, the effusive andesites do not always correspond to the plutonic diorites but partly also to the gabbros.

As the different types of rock, regarded as aggregates of minerals, pass gradually from one to another, transitional types are very common and are often so important as to receive special names. For example, the quartz-syenites and nordmarkites may be interposed between granite and syenite, the tonalites and adamellites between granite and diorite, the monzoaites between syenite and diorite, and the norites and hyperites between diorite and gabbro.

See also

References

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

  • Allègre, Claude J., and Gil Michard. 1974. Introduction to Geochemistry. Geophysics and Astrophysics Monographs, v. 10. Dordrecht: D. Reidel Pub. Co. ISBN 902770497X
  • Faure, Gunter. 1986. Principles of Isotope Geology, 2nd ed. New York: Wiley. ISBN 0471864129
  • Holland, H.D., and K.K. Turekian (eds.). 2003. Treatise on Geochemistry. 10-volume set. New York: Elsevier. ISBN 978-0080437514
  • Levinson, A.A. 1980. Introduction to Exploration Geochemistry, 2nd ed. Wilmette, IL: Applied Pub. ISBN 0915834014
  • Marshall, Clare P., and Rhodes W. Fairbridge (eds.). 2006. Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Berlin: Springer. ISBN 978-1402044960
  • Mason, Brian Harold. 1992. Victor Moritz Goldschmidt: Father of Modern Geochemistry. San Antonio, TX: Geochemical Society. ISBN 094180903X
  • Vernadsky, Vladimir. 2007. Geochemistry and the Biosphere. Edited by Frank B. Salisbury, translated by Olga Barash. Santa Fe: Synergetic Press. ISBN 0907791360

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