Geochemistry is the science that uses the tools and principles of chemistry to explain
the mechanisms behind major geological systems such as the Earth's crust and its oceans.
The realm of geochemistry extends beyond the Earth, encompassing the entire Solar System
and has made important contributions to the understanding of a number of processes including
mantle convection, the formation of planets and the origins of granite and basalt.
History The term geochemistry was first used by the
Swiss-German chemist Christian Friedrich Schönbein in 1838. In his paper, Schönbein predicted
the birth of a new field of study, stating:
"In a word, a comparative geochemistry ought to be launched, before geochemistry can become
geology, and before the mystery of the genesis of our planets and their inorganic matter
may be revealed."
The field began to be realised a short time after Schönbein's work, but his term - 'geochemistry'
- was initially used neither by geologists nor chemists and there was much debate over
which of the two sciences should be the dominant partner. There was little collaboration between
geologists and chemists and the field of geochemistry remained small and unrecognised. In the late
19th Century a Swiss man by the name of Victor Goldschmidt was born, who later became known
as the father of geochemistry. His paper, Geochemische Verteilungsgesetze der Elemente,
on the distribution of elements in nature has been referred to as the start of geochemistry.
During the early 20th Century, a number of geochemists produced work that began to popularise
the field, including Frank Wigglesworth Clarke who had begun to investigate the abundances
of various elements within the Earth and how the quantities were related to atomic weight.
The composition of meteorites and their differences to terrestrial rocks was being investigated
as early as 1850 and in 1901, Oliver C. Farrington hypothesised although there were differences,
that the relative abundances should still be the same. This was the beginnings of the
field of cosmochemistry and has contributed much of what we know about the formation of
the Earth and the Solar System. Subfields
Some subsets of geochemistry are: Isotope geochemistry involves the determination
of the relative and absolute concentrations of the elements and their isotopes in the
earth and on earth's surface. Examination of the distribution and movements
of elements in different parts of the earth and in minerals with the goal to determine
the underlying system of distribution and movement.
Cosmochemistry includes the analysis of the distribution of elements and their isotopes
in the cosmos. Biogeochemistry is the field of study focusing
on the effect of life on the chemistry of the earth.
Organic geochemistry involves the study of the role of processes and compounds that are
derived from living or once-living organisms. Aqueous geochemistry studies the role of various
elements in watersheds, including copper, sulfur, mercury, and how elemental fluxes
are exchanged through atmospheric-terrestrial-aquatic interactions.
Regional, environmental and exploration geochemistry includes 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’. 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. 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 of
the earths curst: 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 and soda 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 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 greater than 66% of silica
are known as felsic rocks, and Quartz is not abundant. In basic rocks it is rare for them
to contain as much silicon, these are referred to as mafic rocks. If Magnesium and Iron are
above 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 alkali's, the greater is the prevalence of plagioclase feldspar as contracted with
soda or potash feldspar. The earth crust is composed of 90% silicate minerals and their
abundance in the earth is as follows; plagioclase feldspar, Alkali feldspar, quartz, pyroxene,
amphiboles, micas, clay minerals, after this the remaining silicate minerals make up another
3% of the earths crust. Only 8% of the earth is composed of non silicate minerals such
as Carbonate, Oxides, and Sulfides. 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. Felsic, intermediate and mafic igneous rocks
Those rocks that contain the most silica, and on crystallizing yield free quartz, form
a group generally designated the "felsic" rocks. Those again that contain least silica
and most magnesia and iron, so that quartz is absent while olivine is usually abundant,
form the "mafic" 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 mafic rocks. Lastly a small sub-group
rich in olivine and without feldspar has been called the "ultramafic" 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 mafic rocks labradorite, anorthite and bytownite
prevail, being rich in lime and poor in silica, potash and soda. Augite is the most common
ferro-magnesian in mafic rocks, but biotite and hornblende are on the whole more frequent
in felsic rocks. Rocks that contain leucite or nepheline, either
partly or a wholly replacing felspar, are not included in this table. They are essentially
of intermediate or of mafic 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.
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 but it has grown up with the growth
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.
See also Important publications in geochemistry
Petrology Tephrochronology
Cosmochemistry Further reading
Holland, H.D., & Turekian, K.K.. Treatise on Geochemistry. 9 Volumes. Elsevier
Marshall, C., & Fairbridge, R.. Encyclopedia of Geochemistry. ISBN 1-4020-4496-8. Berlin:
Springer. Bernard Gunn: The Geochemistry of Igneous
Rocks Gunter Faure, Principles and Applications
of Geochemistry, 2nd edition, Prentice Hall. H.R. Rollinson, Using Geochemical Data: evaluation,
presentation, interpretation. ISBN 978-0-582-06701-1 W.M. White: Geochemistry
EarthDataModels library contains a logical data model and physical implementation of
a Geochemistry database References