Geochemistry

Geochemistry

Geochemistry is the study of the chemical processes that form and shape the earth.

Earth is essentially a large mass of crystalline solids that are constantly subject to physical and chemical interaction with a variety of solutions (e.g., water ) and substances. These interactions allow a multitude of chemical reactions.

It is through geochemical analysis that estimates of the age of Earth are formed. Because radioactive isotopes decay at measurable and constant rates (e.g., half-life ) that are proportional to the number of radioactive atoms remaining in the sample, analysis of rocks and minerals can also provide reasonably accurate determinations of the age of the formations in which they are found. The best measurements obtained via radiometric dating (based on the principles of nuclear reactions) estimate the age of Earth to be four and one half billion years old.

Dating techniques combined with spectroscopic analysis provide clues to unravel Earth's history. Using neutron activation analysis, Nobel Laureate Luis Alvarez discovered the presence of the element iridium when studying samples from the K-T boundary layer (i.e., the layer of sediment laid down at the end of the Cretaceous and beginning of the Tertiary Periods). Fossil evidence shows a mass extinction at the end of the Cretaceous Period , including the extinction of the dinosaurs. The uniform iridium layer—and presence of quartz crystals with shock damage usually associated only with large asteroid impacts or nuclear explosions—advanced the hypothesis that a large asteroid impact caused catastrophic climatic damage that spelled doom for the dinosaurs.

Although hydrogen and helium comprise 99.9% of the atoms in the universe, Earth's gravity is such that these elements readily escape Earth's atmosphere. As a result, the hydrogen found on Earth is found bound to other atoms in molecules.

Geochemistry generally concerns the study of the distribution and cycling of elements in the crust of the earth. Just as the biochemistry of life is centered on the properties and reaction of carbon , the geochemistry of Earth's crust is centered upon silicon . Also important to geochemistry is oxygen . Oxygen is the most abundant element on Earth. Together, oxygen and silicon account for 74% of Earth's crust.

The type of magma (Basaltic, Andesitic or Ryolytic) extruded by volcanoes and fissures (magma is termed lava when at Earth's surface) depends on the percentage of silicon and oxygen present. As the percentage increases, the magma becomes thicker, traps more gas, and is associated with more explosive eruptions.

The eight most common elements found on Earth, by weight, are oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg).

Unlike carbon and biochemical processes where the covalent bond is most common, however, the ionic bond is the most common bond 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. A mineral has a definite (not unique) formula or 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 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 Fe or Mg. Olivine is built by the ionic substitution of Fe and Mg—the atoms are interchangeable because they the same electrical charge and are of similar size—and thus, olivine exists as a range of elemental compositions termed a solid solution series . Olivine can thus be said to be "rich" in iron or rich in magnesium. As magma cools larger atoms such as potassium ions enter into reactions and additional minerals form.

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 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.

The geochemistry involved in many environmental issues has become an increasing important aspect of scientific and political debate. The effects of acid rain are of great concern to geologists not only for the potential damage to the biosphere , but also because acid rain accelerates the weathering process. Rainwater is made acidic as it passes through the atmosphere. Although rain becomes naturally acidic as it contacts nitrogen, oxygen, and carbon dioxide in the atmosphere, many industrial pollutants bring about reactions that bring the acidity of rainwater to dangerous levels. Increased levels of carbon dioxide from industrial pollution can increase the formation of carbonic acid. The rain also becomes more acidic. Precipitation of this "acid rain" adversely affects both geological and biological systems.

According to plate tectonic theory, the crust (lithosphere ) of Earth is divided into shifting plates. Geochemical analysis of Earth's tectonic plates reveals a continental crust that is older, thicker and more granite-like than the younger, thinner oceanic crusts made of basaltic (iron, magnesium) materials.

See also Atomic mass and weight; Atomic number; Atomic structure; Big Bang theory; Chemical bonds and physical properties; Chemical elements; Geologic time