Geochemistry: Definition, Applications and Sample Questions

Geochemistry is that branch of science that establishes a relation between chemistry and geology and uses this relation as an investigative way to explore more about the earth and the environment. The earth has a complex chemical structure and so does the cosmos surrounding it. Geochemistry applies chemistry to study the numerous geological phenomena at the atomic and subatomic levels. This helps in determining the distribution, concentration, origin and evolution of chemical elements which form the structure of the earth and the surrounding cosmos.

Keyterms: Geochemistry, Geology, Environment, Subatomic levels, Cosmos, Evolution, Earth, Geological structure, Earth’s crust, Oceans, Solar system

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What is Geochemistry?

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Geochemistry can be defined as the branch of chemistry which studies the geological structure of the earth and the surrounding universe at atomic and subatomic levels to determine the different aspects of major geological mechanisms such as the earth’s crust, oceans, the solar system, etc. Geochemistry helps scientists in analysing the chemical composition of natural celestial bodies like meteorites and other planets of the solar system.

Geochemistry

Geochemistry provides the necessary information about the origin and composition of the earth’s crust, mantle and core. It explores several chemical and mineral transformations that occur in the rocks, for example, the metamorphosis of rocks, weathering of rocks, etc. Geochemistry has successfully contributed towards understanding some of the very important geological mechanisms like mantle convection, the origin of basalt and granite rocks, formation of the planets of the solar universe.

Rock Cycle

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Subfields of Geochemistry

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Some important subfields of geochemistry are :

1. Aqueous Geochemistry
2. Biogeochemistry
3. Organic Geochemistry
4. Photogeochemistry
5. Isotopic Geochemistry
6. Regional Geochemistry
7. Cosmochemistry

Aqueous Geochemistry 

Aqueous geochemistry explores the behaviour of chemical elements present in the water bodies. Elements like Copper, Sulphur and Mercury have a substantial presence in aquatic systems. Aqueous geochemistry studies the interaction between terrestrial and aquatic systems and investigates the exchange of elemental flux across terrestrial and aquatic boundaries. 

Aqueous Geochemistry

Biogeochemistry

Biogeochemistry studies the elemental transaction happening between living and non-living systems. It focuses on the origin of life on earth and the subsequent changes in the chemical environment of earth. 

Biogeochemistry

Organic Geochemistry

Organic geochemistry studies the behaviour of organic materials present on earth. These organic materials have been derived from living or dead organisms. It also explores the transformation of carbonaceous deposits in rocks and fossil fuels. A number of organic chemical reactions are the underlying forces behind major geological mechanisms like rock formation, rock weathering, soil formation.

Organic Geochemistry

Photogeochemistry

Photogeochemistry uses the concepts of photochemistry and geochemistry to study and analyse the chemical reactions that are induced by light energy and lead to the geological transformation of minerals. Sunlight induced chemical changes occur in the solid components like rocks and soil, aquatic components like dissolved organic matter in aquatic bodies and atmospheric components like aerosols and gases. 

Photogeochemistry

These reactions are outside the realm of living things. The study of photo geochemistry is primarily focused on the chemical reactions of natural organic or synthesised inorganic materials.

Isotopic Geochemistry

Isotopic geochemistry gives a detailed analysis of relative concentrations of all the chemical elements and their isotopes and the variations in their natural abundance using mass spectrometry. Isotopic geochemistry aims to find more information about the origin of rocks, water bodies and transboundary elemental transactions between them. Stable isotopic geochemistry and radiogenic isotopic geochemistry are two fields of study under isotopic geochemistry.

Marine Isotopic Geochemistry

Regional Geochemistry

Regional geochemistry aims at mapping the geochemical resources which are present on the earth’s surface. This study involves the survey of vast areas of earth exploring soil, water, vegetation, natural bedrock, etc. On the basis of these surveys, samples are collected. Then by applying chemical analysis methods the spatial distribution of geochemical elements on earth is extracted. 

Regional Geochemistry

This gives an overview of the regional abundance of minerals. Using Regional geochemistry scientists are able to determine the variation in abundance of minerals on a scale of thousands of kilometres.

Cosmochemistry 

Cosmochemistry is also called chemical cosmology. Cosmochemistry studies the chemical composition of elements present in the universe and aims to explore the geochemical processes involved in their origin, development and transformation. 

Cosmochemistry

Under its realm are celestial bodies, meteorites, asteroids of the solar system. On the basis of this study, researchers compile data regarding the abundance and concentration of cosmic elements in the universe. Victor Goldschmidt was one of the first proponents of this field of study.

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Earth and Its Mineral Constituents

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The earth comprises hundreds of minerals in its crust, mantle and core. The variety and abundance of minerals on earth make it a unique composition. Elements like oxygen, aluminium, silicon, calcium, sodium and potassium make up around 98 percent of the total crust. The elements mentioned above are rarely found in the elemental state and are mostly present in a combined state. The following table shows the element-wise presence in the earth’s crust :

Combined state of different elements: Minerals

The above-mentioned elements are mostly present in compound forms. The following table presents the abundance of major compounds in the earth’s crust :

The earth’s crust has around 2000 types of minerals. The most abundant are feldspar, quartz, pyroxenes, mica and olivine. The magma present in the interior of the earth is the source of all these minerals. These minerals can be organic or naturally occurring and inorganic. Minerals like haematite are metallic minerals. Some of the major minerals are discussed below :

Feldspar

• The basic constituents of this mineral are silicon and oxygen. Other elements like sodium, potassium, calcium and aluminium also have a varied presence across different varieties of feldspar.
• It is light pink in colour and is majorly used in the ceramics industry.

Feldspar

Quartz

• The major constituent of quartz is silica. It is a hard mineral.
• Widely used in radios and radars.

Quartz

Pyroxene

• The basic constituents are calcium, iron, silicon, magnesium and aluminium.
• It makes up 10 % of the earth’s crust. 

Pyroxene

Mica

• Elements like potassium, silicon, iron, magnesium are its basic constituents.
• Commonly present in metamorphic rocks.
• Wide application in the electronic industry.

Mica

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Scope of Stable Isotope Geochemistry

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Stable isotope geochemistry studies the isotopic variations in the geochemical elements present on the earth. The difference in mass of molecules either enriches or impoverishes certain isotopic species.

Different isotopes of the same element can be present in the same compound. For example, CaCO₃. In the case of calcium carbonate, two isotopes of oxygen can be present i.e. oxygen-16 and oxygen-18. The proportion of their presence is determined by the temperature of seawater.

Stable Isotope Chemistry

Stable isotopic geochemistry also helps in the carbon-dating process by analyzing the isotopic composition of fossils. In this way, isotopic geochemistry helps in the quantification of the geological time scale. The C-14 isotope of carbon is extremely useful in analyzing the origin and transformation of biological materials of the past. The major part of stable isotope geochemistry deals in the isotopes of the elements: Hydrogen, Carbon, Oxygen, Nitrogen and Sulfur. 

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Applications of Geochemistry

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• Stable isotopic geochemistry also helps in the carbon-dating process by analyzing the isotopic composition of fossils.
• Cosmochemistry studies the chemical composition of elements present in the universe and aims to explore the geochemical processes involved in their origin, development and transformation.

Carbon Dating

• Photogeochemistry uses the concepts of photochemistry and geochemistry to study and analyse the chemical reactions that are induced by light energy and lead to the geological transformation of minerals.
• Geochemistry traces metals in food chains, soil and water. For example Copper and Cobalt are a necessity in forage plants, if they are deficient, that will lead to diseases in grazing animals and can in turn affect human health.
• Geochemistry has successfully contributed towards understanding some of the very important geological mechanisms like mantle convection, the origin of basalt and granite rocks, formation of the planets of the solar universe.
• Geochemistry contributes to the exploration of mineral deposits. It aims to investigate the processes(metamorphic, hydrothermal, hydraulic, etc) that lead to the production of minerals.

Origin of Different Rocks

• Mass spectrometry methods are employed under geochemistry to analyze the identification and characterization of minerals.
• Geochemistry helps to understand environmental disasters. For example volcanic eruptions.
• Geochemistry has applications in archaeology and anthropology. It is used to investigate the concentration and consumption of metals on a regional basis. It has led to path-breaking discoveries in regions of South Africa.

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Things to Remember

• Victor Goldschmidt is known as the father of Geochemistry.
• Cosmochemistry studies the chemical composition of elements present in the universe and aims to explore the geochemical processes involved in their origin, development and transformation.
• Geochemistry has successfully contributed towards understanding some of the very important geological mechanisms like mantle convection, origin of basalt and granite rocks, formation of the planets of the solar universe.
• Elements like oxygen, aluminium, silicon, calcium, sodium and potassium make up around 98 percent of the total crust. 
• Geochemistry applies chemistry to study the numerous geological phenomena at the atomic and subatomic levels.
• Regional geochemistry aims at mapping the geochemical resources which are present on the earth’s surface. This study involves the survey of vast areas of earth exploring soil, water, vegetation, natural bedrock, etc.
• Aqueous geochemistry studies the interaction between terrestrial and aquatic systems and investigates the exchange of elemental flux across terrestrial and aquatic boundaries. 

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