Geochemistry

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.