Share on Facebook Share on Twitter Email
Answers.com

geochemistry

 
American Heritage Dictionary:

ge·o·chem·is·try

('ō-kĕm'ĭ-strē) pronunciation
n.
The chemistry of the composition and alterations of the solid matter of the earth or a celestial body.

geochemical ge'o·chem'i·cal (-ĭ-kəl) adj.
geochemically ge'o·chem'i·cal·ly adv.
geochemist ge'o·chem'ist n.

Search unanswered questions...
Enter a question here...
Search: All sources Community Q&A Reference topics

Scientific discipline dealing with the relative abundance, distribution, and migration of the Earth's chemical elements and their isotopes. Historically, geochemistry was concerned primarily with defining elemental abundances in minerals and rocks. Modern geochemical research also includes study of the continual recycling of the Earth's constituent materials through geologic processes, the cyclic flow of individual elements (and their compounds) between living and nonliving systems, and certain areas of cosmology.

For more information on geochemistry, visit Britannica.com.

A field that encompasses the investigation of the chemical composition of the Earth, other planets, and the solar system and universe as a whole, as well as the chemical processes that occur within them. The discipline is large and very important because basic knowledge about the chemical processes involved is critical for understanding subjects as diverse as the formation of economically valuable ore deposits, safe disposal of toxic wastes, and variations in the Earth's climate.

Isotope geochemistry is based on the fact that the isotopic compositions of various chemical elements may reveal information about the age, history, and origin of terrestrial and extraterrestrial materials. Isotopes of an element share the same chemical properties but have slightly different nuclear makeups and therefore different masses. Some naturally occurring isotopes are radioactive and decay at known rates to form daughter isotopes of another element; for example, radioactive uranium isotopes decay to stable isotopes of lead. Radioactive decay is the basis of geochronology, or age determination: the age of a sample can be found by measuring its content of the daughter isotope. Both radioactive decay and the processes that enrich or deplete materials in certain isotopes cause different parts of the Earth and solar system to have different, characteristic isotopic compositions for some elements. These differences serve as fingerprints for tracing the origins of, and characterizing the interactions between, various geochemical reservoirs. See also Dating methods; Elements, geochemical distribution of; Geochronometry; Isotope; Lead isotopes (geochemistry).

Cosmochemistry deals with nonearthly materials. Typically, cosmochemists use the same kinds of analytical and theoretical approaches as other geochemists but apply them to problems involving the origin and history of meteorites, the formation of the solar system, the chemical processes on other planets, and the ultimate origin of the elements themselves in stars. See also Cosmochemistry; Meteorite; Solar system.

Organic geochemistry deals with carbon-containing compounds, largely those produced by living organisms. These are widely dispersed in the outer part of the Earth—in the oceans, the atmosphere, soil, and sedimentary rocks. Organic geochemistry is important for understanding many of the chemical cycles that occur on Earth because biology often plays a major role. Organic geochemists are also active in investigating such areas as the origin of life, the formation of some types of ore deposits that may be biologically mediated, and the origin of coal, petroleum, and natural gas. See also Biogeochemistry; Coal; Natural gas; Organic geochemistry; Petroleum; Prebiotic organic synthesis.

In recent years there has been widespread application of geochemical techniques to problems in paleoclimatology and paleoceanography. In this approach, ocean sediments, sedimentary rocks on land, ice cores, and other continuous records of the Earth's history are analyzed for fossil chemical evidence of past climates or seawater composition. As in most areas of geochemistry, precise and accurate analytical methods for determining the isotopic and elemental composition of the samples are critical. See also Earth sciences; Paleoceanography; Paleoclimatology.


Columbia Encyclopedia:

geochemistry

Top
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.

Bibliography

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


Random House Word Menu:

categories related to 'geochemistry'

Top
Random House Word Menu by Stephen Glazier
For a list of words related to geochemistry, see:

Wikipedia on Answers.com:

Geochemistry

Top

The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and their interaction with the hydrosphere and the atmosphere.

Some subsets of geochemistry are:

  1. Isotope geochemistry:Determination of the relative and absolute concentrations of the elements and their isotopes in the earth and on earth's surface.
  2. Examination of the distribution and movements of elements in different parts of the earth (crust, mantle, hydrosphere etc.) and in minerals with the goal to determine the underlying system of distribution and movement.
  3. Cosmochemistry: Analysis of the distribution of elements and their isotopes in the cosmos.
  4. Biogeochemistry: Field of study focusing on the effect of life on the chemistry of the earth.
  5. Organic geochemistry: A study of the role of processes and compounds that are derived from living or once-living organisms.
  6. Aqueous geochemistry: Understanding the role of various elements in watersheds, including copper, sulfur, mercury, and how elemental fluxes are exchanged through atmospheric-terrestrial-aquatic interactions.
  7. Regional, environmental and exploration geochemistry: Applications to environmental, hydrological and mineral exploration studies.

Victor Goldschmidt is considered by most to be the father of modern geochemistry and the ideas of the subject were formed by him in a series of publications from 1922 under the title ‘Geochemische Verteilungsgesetze der Elemente’ (geochemical laws of distribution of the elements).

Contents

Chemical characteristics

The more common rock constituents are nearly all oxides; chlorides, sulfides and fluorides are the only important exceptions to this and their total amount in any rock is usually much less than 1%. F. W. Clarke has calculated that a little more than 47% of the Earth's crust consists of oxygen. It occurs principally in combination as oxides, of which the chief are silica, alumina, iron oxides, and various carbonates (calcium carbonate, magnesium carbonate, sodium carbonate, and potassium carbonate). The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1672 analyses of numerous kinds of rocks Clarke arrived at the following as 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, P2O5=0.22, total 99.22%). All the other constituents occur only in very small quantities, usually much less than 1%.

These oxides combine in a haphazard way. For example, potash (potassium carbonate) and soda (sodium carbonate) combine to produce feldspars. In some cases they may take other forms, such as nepheline, leucite, and muscovite, but in the great majority of instances they are found as feldspar. Phosphoric acid with lime (calcium carbonate) forms apatite. Titanium dioxide with ferrous oxide gives rise to ilmenite. Part of the lime forms lime feldspar. Magnesium carbonate and iron oxides with silica crystallize as olivine or enstatite, or with alumina and lime form the complex ferro-magnesian silicates of which the pyroxenes, amphiboles, and biotites are the chief. Any excess of silica above what is required to neutralize the bases will separate out as quartz; excess of alumina crystallizes as corundum. These must be regarded only as general tendencies. It is possible, by rock analysis, to say approximately what minerals the rock contains, but there are numerous exceptions to any rule.

Mineral constitution

Hence we may say that except in acid or siliceous rocks containing 66% of silica and over, quartz will not be abundant. In basic rocks (containing 20% of silica or less) it is rare and accidental. If magnesia and iron be above the average while silica is low, olivine may be expected; where silica is present in greater quantity over ferro-magnesian minerals, such as augite, hornblende, enstatite or biotite, occur rather than olivine. Unless potash is high and silica relatively low, leucite will not be present, for leucite does not occur with free quartz. Nepheline, likewise, is usually found in rocks with much soda and comparatively little silica. With high alkalis, soda-bearing pyroxenes and amphiboles may be present. The lower the percentage of silica and the alkalis, the greater is the prevalence of calcium feldspar as contracted with soda or potash feldspar. Clarke has calculated the relative abundance of the principal rock-forming minerals with the following results: apatite=0.6, titanium minerals=1.5, quartz=12.0, feldspars=59.5, biotite=3.8, hornblende and pyroxene=16.8, total=94.2%. This, however, can only be a rough approximation.

The other determining factor, namely the physical conditions attending consolidation, plays on the whole a smaller part, yet is by no means negligible, as a few instances will prove. Certain minerals are practically confined to deep-seated intrusive rocks, e.g., microcline, muscovite, diallage. Leucite is very rare in plutonic masses; many minerals have special peculiarities in microscopic character according to whether they crystallized in depth or near the surface, e.g., hypersthene, orthoclase, quartz. There are some curious instances of rocks having the same chemical composition, but consisting of entirely different minerals, e.g., the hornblendite of Gran, in Norway, which contains only hornblende, has the same composition as some of the camptonites of the same locality that contain feldspar and hornblende of a different variety. In this connection we may repeat what has been said above about the corrosion of porphyritic minerals in igneous rocks. In rhyolites and trachytes, early crystals of hornblende and biotite may be found in great numbers partially converted into augite and magnetite. Hornblende and biotite were stable under the pressures and other conditions below the surface, but unstable at higher levels. In the ground-mass of these rocks, augite is almost universally present. But the plutonic representatives of the same magma, granite and syenite contain biotite and hornblende far more commonly than augite.

Acid, intermediate and basic igneous rocks

Those rocks that contain the most silica, and on crystallizing yield free quartz, form a group generally designated the "acid" rocks. Those again that contain least silica and most magnesia and iron, so that quartz is absent while olivine is usually abundant, form the "basic" group. The "intermediate" rocks include those 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 sub-group rich in olivine and without feldspar has been called the "ultrabasic" rocks. They have very low percentages of silica but much iron and magnesia.

Except these last, practically all rocks contain felspars or feldspathoid minerals. In the acid rocks the common feldspars are orthoclase, perthite, microcline, and oligoclase—all having much silica and alkalis. In the basic rocks labradorite, anorthite and bytownite prevail, being rich in lime and poor in silica, potash and soda. Augite is the commonest ferro-magnesian of the basic rocks, but biotite and hornblende are on the whole more frequent in the acid.

Commonest Minerals Acid Intermediate Mafic Ultramafic
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 felspar, are not included in this table. They are essentially of intermediate or of basic character. We might in consequence regard them as varieties of syenite, diorite, gabbro, etc., in which feldspathoid minerals occur, and indeed there are many transitions between syenites of ordinary type and nepheline — or leucite — syenite, and between gabbro or dolerite and theralite or essexite. But, as many minerals develop in these "alkali" rocks that 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

This classification is based essentially on the mineralogical constitution of the igneous rocks. Any chemical distinctions between the different groups, though implied, are relegated to a subordinate position. It is admittedly artificial by it has grown up with the grown of the science and is still adopted as the basis on which more minute subdivisions are erected. The subdivisions are by no means of equal value. The syenites, for example, 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 kinds of rock, regarded as aggregates of minerals, pass gradually into one another, transitional types are very common and are often so important as to receive special names. 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, norites and hyperites between diorite and gabbro, and so on.[1]

See also

Further reading

References

  1. ^  This article incorporates text from a publication now in the public domainChisholm, Hugh, ed (1911). "Petrology". Encyclopædia Britannica (11th ed.). Cambridge University Press. 

 
 
Related topics:
biogeochemistry (geochemistry)
desilication (geochemistry)
base exchange (geochemistry)

Related answers:
What you need to become a geochemistry? Read answer...
Is geochemistry the combination of geology and chemistry? Read answer...
What is a sentence with the word geochemistry in it? Read answer...

Help us answer these:
What is the sudy of geochemistry?
What is the frontier of Geochemistry?
Where did geochemistry originate?

Post a question - any question - to the WikiAnswers community:

 

Copyrights:

American Heritage Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 1994-2012 Encyclopædia Britannica, Inc. All rights reserved.  Read more
McGraw-Hill Science & Technology Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2012, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/ Read more
Random House Word Menu. © 2010 Write Brothers Inc. Word Menu is a registered trademark of the Estate of Stephen Glazier. Write Brothers Inc. All rights reserved.  Read more
Wikipedia on Answers.com. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article Geochemistry Read more

Follow us
Facebook Twitter
YouTube

Mentioned in

» More» More