The geochemistry ( Greek γεω- geo- , the Earth concerning '[to γῆ ge ' earth '], and chemistry ) is a branch of chemistry that deals with the material construction of the distribution, stability and the cycle of chemical elements and their isotopes in minerals , rocks , soil , water , earth's atmosphere and biosphere . It is the scientific discipline that combines geology and chemistry. It shares the object of investigation with geology and the investigation methods with chemistry.
Until the late 19th century, a descriptive approach was pursued in the geosciences, which sought to understand rocks and minerals according to their external properties, but did not or hardly included the material basis and their chemical dynamics. Understanding these dynamics is essential, however, because many questions can only be answered by geochemical approaches.
The history of modern geochemistry, of which Victor Moritz Goldschmidt , Wladimir Iwanowitsch Wernadski , Frank Wigglesworth Clarke and Alfred Treibs were among the founders in the early to mid-20th century, is therefore closely linked to that of geology and mineralogy . The term itself goes back to the Swiss chemist Christian Friedrich Schönbein (1838). The work of Karl Gustav Bischof (1846), Justus Roth (1818–1892; 1859) and James David Forbes (1868) represented important stages on the way to a modern understanding of geochemistry .
In modern geochemistry, the subject is divided into two parts. On the one hand there is the investigation of metamorphic and igneous rocks, whereby the main focus is on their trace element content and (mostly radiogenic) isotope ratios, with the aim of being able to make statements about age ( geochronology ) and formation conditions ( geothermobarometry ). In the area of the reconstruction of the earliest geological history, there is an overlap with planetology and cosmochemistry . On the other hand there is the investigation of sediments, water, soils, living beings and the air, with the investigation of stable isotopes and the speciation of elements playing a prominent role. At this end of the spectrum of geochemistry, biogeochemistry , i.e. the study of the influence of organisms on the chemistry of the earth, forms the transition to biochemistry and biology .
For the analysis of liquid samples, ion exchange chromatography is often used to determine the main elements , optical emission spectrometry with inductively coupled plasma (ICP-OES) for trace elements and mass spectrometry with inductively coupled plasma (ICP-MS) for ultra- trace elements . The latter can also be used to measure the frequency of different isotopes in a sample. Through the coupling with a laser, solid samples can also be examined with the ICP-MS, with the laser removing material from the sample surface. Another way to measure the chemical composition of solid samples directly is electron beam microanalysis . Often solid samples are also subjected to a digestion and either melted or dissolved. The solidified orodispersible tablet can then be examined using X-ray fluorescence analysis, while the entire range of the above-mentioned methods is available for solutions.
In addition to these standard methods, there are other methods for special questions: Mössbauer spectroscopy to differentiate between divalent and trivalent iron , electron spin resonance for the detection of low concentrations of paramagnetic ions in minerals, X-ray absorption spectroscopy and atomic force microscopy for the chemical investigation of surfaces, Raman spectroscopy and infrared spectroscopy for Detection of certain bonds and the elements involved in them as well as the neutron activation analysis for extremely low concentrations.
- Geochemistry of the lithosphere
The content and distribution of elements in a mineral provide information about the history of the rock formation, including the pressure and temperature conditions at the time of formation ( geothermobarometry ). Many rock classifications are based on geochemical data. Examples are the TAS diagram (Total Alkali Silica) for glassy volcanic rocks and the subdivision of granites into S-type granites and I-type granites . The concentrations of lanthanides are very often used to determine the formation conditions and to classify rocks.
- Geochemistry of the hydrosphere
Hydrogeochemistry examines the water quality of surface and groundwater , the water cycle and the interactions between water and minerals.
- Geochemistry of the Earth's Atmosphere
Important topics in atmospheric chemistry are the greenhouse effect , air pollution from fine dust and acid rain .
Stable isotopes provide information about areas of formation, weathering processes and transport processes of rocks, ores and water; radiogenic isotopes enable the age of minerals and rocks to be determined ( geochronology ).
The investigation of meteorites provides information about the formation of the universe , the solar system and the earth .
The basics of geochemistry are taught in many geoscientific bachelor's degree programs (e.g. "Geosciences", "Geology / Mineralogy"). In-depth knowledge can be acquired in the master’s degree in “Geomaterials and Geochemistry” or in geochemical specializations, mostly mineralogically oriented degree programs. In German university policy, geochemistry is classified as a minor subject ; it is recorded by the “Small Subjects Unit” together with mineralogy, petrology and similar fields.
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- Marshall, CP; Fairbridge, RW (1999): Encyclopedia of geochemistry. Encyclopedia of Earth Sciences Series. Springer-Verlag, ISBN 0-412-75500-9
- Mason, B. & Moore, C. (1985): Principles of Geochemistry . 2nd Edition. Spectrum Academic Publishing House, ISBN 978-3-8274-1262-1
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