Geochemical Analysis

Geochemical analysis became important in the nineteenth and twentieth centuries, when chemists first began investigating the compounds that formed naturally in the earth, air, and water. Much of this early work was credited to a chemist named V. M. Goldschmidt, who with his students created detailed charts of the chemical breakdown of common compounds, mainly igneous rocks. He also created a series of guidelines known collectively as "Goldschmidt's Rules" for understanding the different ways in which elements interact to form different types of rock. Scientists have expanded on Goldschmidt's program, forming a series of disciplines that help them predict and interpret the chemical composition through time of this planet, other objects in the solar system (including planets), and their constituent ingredients. Goldschmidt based his analysis of chemical behavior on two separate items: size and electrical charge. Later scientists have added radiation to the process of geochemical analysis, grouping elements by their radioactive and stable isotopes. Isotopic analysis can give clues to the place of origin of the compound, and the environment in which it was first put together. Isotopes are also used to determine the age of a compound, and the study of the process through which they decay from one form to another is known as geochronology. Astronomers have discovered certain isotopes in compounds located in celestial bodies—like supernovae—which have relatively short half-lives, and they use these substances to help date the formation of the universe.

One of the most commercially popular subfields of modern geochemistry is geochemical prospecting, usually in order to locate metals like uranium or hydrocarbons like petroleum. The methodology for geochemical prospecting was pioneered in Europe and the Soviet Union during the 1930s. It was taken up by prospectors in the United States after World War II. Prospectors find that the most profitable way to search for valuable rock and mineral samples is to look in areas that have undergone extensive weathering, especially the beds of streams. Using their knowledge of weathering and dispersion patterns, these scientists examine samples drawn from areas where streams intersect each other and from places where fault lines have caused slippages of the local geography to detect the presence of valuable substances. They also can detect minerals that have undergone chemical decomposition by analyzing the surrounding water and sand and silt deposits for trace remnants, which form a characteristic spread known as a secondary dispersion halo. By examining the characteristics of these elements and comparing the results to a series of known features in areas like valency, ionic size, and type of chemical bond, geochemists can discover if commercial valuable minerals are present in the area. Other elements, especially volatile ones like chlorine, fluorine, sulfur, carbon dioxide, and water, also serve as indicators of elements that may be found in the area. Prospectors searching specifically for petroleum look for a polymer called kerogen, thought to be a substance falling between the original organic material that makes up petroleum or natural gas, and the final product, in soil and rock samples. The presence of radon, which can be detected relatively easily because of its characteristic alpha radioactivity, in the water of streams is often an indicator of uranium deposits.

Geochemists have also developed a variety of innovative and cost-saving ways of performing geochemical analysis without requiring to be in direct contact with the rocks and minerals they are examining. One relatively common way is to examine the surface flora and fauna for traces of chemicals or metals. Certain plants growing in contaminated areas develop characteristic diseases, such as chlorosis or nongenetic dwarfism. Contamination can also be detected from chemical residue collected in the internal organs of fish, molluscs, and insects. Some geochemists have even used dogs to recognize and locate minerals that are found in combination with sulfur compounds by teaching them to sniff out the gasses released in the oxidation process. Prospectors also use aerial surveys, computer mapping and modeling, and atomic absorption spectrometry to gather clues as to where the minerals they are seeking can be found. Apparatuses that can record gamma radiation are mounted in airplanes and used to locate radioactive minerals.