Geochemistry

Geochemistry is the science or study of the chemistry of the earth . Geochemists who practice this science are interested in the origin of chemical elements, their evolution , the classes and many divisions of minerals and rocks and how they are created and changed by earth processes, and the circulation of chemical elements through all parts of the earth including the atmosphere and biological forms.

The circulation of elements in nature has many practical applications. Understanding the distribution of chemical isotopes and their stability (or instability) helps in fields as varied as age-dating in archaeology and medical uses of radioactive isotopes. Some significant chemical elements like carbon , phosphorus , nitrogen , and sulfur have geochemical cycles that are indicators of environmental contamination or the need to rotate crops in fields.

Geochemistry has many subdivisions. Inorganic geochemistry explains the relationships and cycles of the elements and their distribution throughout the structure of the earth and their means of moving by thermodynamics and kinetics. Exploration geochemistry (also called geochemical prospecting) uses geochemical principles to locate ore bodies, mineral fields, groundwater supplies, and oil and gas fields. Organic geochemistry uses the chemical indicators associated with life forms to trace human habitation as well as plant and animal activity on Earth. It has been important in understanding the paleoclimate , paleooceanography, and primordial life and life's evolution. Sedimentary geochemistry interprets what is known from hard rock geochemistry in soil and other sediments and their erosion , deposition patterns, and metamorphosis into rock. Environmental geochemistry is the newest branch of the science and came into prominence in the 1980s when environmental concerns made the tracking of chemicals in organic tissues, groundwater, surface water , the marine environment, soil, and rock important to scientists, engineers, and government agencies responsible for the public's well-being.

History of the science of geochemistry

Geochemistry did not come into its own as a science until the 1800s. The discoveries of hydrogen and oxygen (two of the most important elements needed for life on earth) in 1766 by Henry Cavendish and in 1770 by Joseph Priestley, respectively, opened the door for understanding chemical elements and the concept of the atom. Russia stakes claim to the founder of geochemistry, V. I. Vernadsky. Vernadsky was an expert in mineralogy and was the first to relate chemical elements to the formation of minerals in nature. Also in the 1800s, pioneering geochemists discovered how to produce salt from seawater and identified other elements and compounds like ozone , which helped in the understanding of the creation of life on earth and the chemical requirements for maintaining it. The first American geochemist was F. W. Clarke, who became the chief chemist of the United States Geological Survey shortly after it was founded in 1884.

Characteristics and processes

Just as the biochemistry of life is centered on the properties and reaction of carbon, the geochemistry of the Earth's crust is centered upon silicon. (Si). Also important to geochemistry is oxygen, the most abundant element on earth. Together, oxygen and silicon account for 74% of the Earth's crust.

Unlike carbon and biochemical processes where the covalent bond is most common, ionic bonds are typically found in geology . Accordingly, silicon generally becomes a cation and will donate four electrons to achieve a noble gas configuration. In quartz, each silicon atom is coordinated to four oxygen atoms . Quartz crystals are silicon atoms surrounded by a tetrahedron of oxygen atoms linked at shared corners.

Rocks are aggregates of minerals and minerals are composed of elements. All minerals have a definite structure and composition. Diamonds and graphite are minerals that are polymorphs (many forms) of carbon. Although they are both composed only of carbon, diamonds and graphite have very different structures and properties. The types of bonds in minerals can affect the properties and characteristics of minerals.

Pressure and temperature also affect the structure of minerals. Temperature can determine which ions can form or remain stable enough to enter into chemical reactions . Olivine (Fe, Mg)2 SiO4), for example, is the only solid that will form at 1,800°C. According to Olivine's formula, it must be composed of two atoms of either iron or magnesium . The atoms are interchangeable because they carry the same electrical charge and are of similar size, thus, Olivine exists as a range of compositions termed a solid solution series. Depending upon the ionic substitution of iron or magnesium, Olivine is said to be either rich in iron or rich in magnesium.

The determination of the chemical composition of rocks involves the crushing and breakdown of rocks until they are in small enough pieces that decomposition by hot acids (hydrofluoric, nitric, hydrochloric, and perchloric acids) allows the elements present to enter into a solution for analysis. Other techniques involve the high temperature fusion of powdered inorganic reagent (flux) and the rock. After melting the sample , techniques such as x-ray fluorescence spectrometry may be used to determine which elements are present.

Chemical and mechanical weathering break down rock through natural processes. Chemical weathering of rock requires water and air. The basic chemical reactions in the weathering process include solution (disrupted ionic bonds), hydration, hydrolysis , and oxidation.

Geochemistry for the future

Of all the important work done by geochemists, improving our understanding of life on Earth may be the highest priority. In their mastery of the cycles of elements, geochemists can analyze other cycles such as the delicate balance between the atmosphere and the hydrosphere that produces the biosphere , or the portions of the earth either on land or in the water and air where life flourishes. New advances and understanding are broadening public awareness of the need to preserve the atmosphere and the oceans and our need to see the earth as a complicated mechanism.