All things in the universe consist of a combination of elements in specific orders and following certain "natural laws". Rocks are no exceptions. They consist of arrangements of elements in specific chemical groups, forming neutral charged compounds. To understand rocks, a basic understanding of chemistry is needed. The following is a very generalized and simplified summary of chemistry to understand geologic concepts.
Matter comes in three forms (solids, liquids and gases) that can be broken down into smaller particles known as atoms. An atom is the smallest particle of an element that retain all its chemical properties.
Atoms are composed of a nucleus surrounded by constantly moving electrons. The nucleus is composed of protons and neutrons. The protons have a positive (+) charge. (Elements of different types will have different numbers of protons.) The neutrons have a neutral charge and add "weight" and "stability" to the atomic structure.
The outer portion of the atom contains various levels that are filled by the moving electrons. These electrons have a negative (-) charge. For an atom to have a neutral charged, it must have the same number of electrons in the outer "shells" as there are protons in the nucleus. But the electrons' energy levels may not fit this "configuration". Often atoms loss or gain electrons to fill outer shells and stabilize the energy levels. This produces a charged atom called an ion. In geology, the "magic" configuration is eight electrons in the outer shell and the atoms associated with minerals will often have charges based on this configuration.
There are over 100 known elements, only a few of which are important in introductory geology. This includes the eight most common elements on the earth's crust:
|Element ||Symbol ||Charge || |
|Oxygen ||O ||-2 ||(the only negatively charged element in this group) |
|Silicon ||Si ||+4 || |
|Aluminum ||Al ||+3 || |
|Iron ||Fe ||+2, +3 || |
|Calcium ||Ca ||+2 ||(Let's consider the positively |
|Sodium ||Na ||+1 ||charged elements as "metals" |
|Magnesium ||Mg ||+2 ||to simplify further chemistry.) |
|Potassium ||K ||+1 || |
Elements are represented by one, two, or three-letter abbreviations. The symbol consists of a capital letter, followed by small letters (when appropriate). The symbol is unique for each element. Symbols are surrounded by four positions which represent important properties of that element to a chemist.
The number at:
- --(mmm) represents the atoms mass (protons plus neutrons);
--(nnnn) represents the atomic number (number of protons);
--(aaaa) is the number of that atom present within any given chemical formula; and
--(cccc) is the charge of that atom as an ion (negative symbol must be supplied).
CO=Carbon and Oxygen (one of each) and does not equal Co (Cobalt)
Atoms combine with other atoms to form molecules and compounds through various chemical means. This combination, known as bonding, can occur in several ways. Geologist are concerned with only four varieties: Ionic bonds, covalent bonds, metallic bonds and Van der Waals bonds. The resulting compound in geology must have a neutral charge.
In ionic bonds, an atom that loses electrons (positive charge) joins with an atom that gains electrons (negative charge).
Ex.: Sodium Chloride (Geology Name: Halite)
- Na (+1 charge) + Cl (- 1 charge)=NaCl
Atoms that can either gain or lose electrons will often bond covalently. This is where similar atoms share electrons between themselves.
Ex.: Carbon (Geology Names: Graphite and Diamond)
Metallic bonds occur with "pure" metallic elements. The electrons are not confined to a single atom and "roam" freely within the piece of metal. This allows special properties such as magnetism, malleability, and conductivity.
Ex.: Iron, Aluminum, Gold, Silver
Van der Waals bond is a weak bond that occurs in several mineral "families". This bond occurs between neutral areas and acts similar to "static electricity", holding the mineral together until something interferes.
Ex.: Micas and Clays
Based on availability and chemical reactivity, there are a limited number of combinations, in geology, that occur when the elements bond together. These combinations can be lumped into groups that behave in chemically similar ways. There are seven common chemical groups in geology.
Elemental Chemical Group
Minerals consisting of pure elements are grouped into a single chemical group. This group can be divided into two areas: metallic and nonmetallic types. Metallic types include gold (Au), silver (Ag), copper (Cu) and other economic ore minerals. Nonmetallics include sulfur (S), and carbon (in the form of graphite and diamonds).
Oxide Chemical Group
Oxide minerals consist of negative oxygen ions bonded to one or more positive "metallic" ions.
Ex.: Aluminum Oxide (Geologic Name: Corundum); Iron Oxides (Geologic Names: Hematite and Magnetite)
Sulfide Chemical Group
Minerals containing negative sulfur ions bonded to one or more positive "metallic" ions are known as sulfides.
Ex.: Iron Sulfide (Geologic Name: Pyrite)
Halide Chemical Group
Halides are a chemical group that include chlorides, bromides and iodides as a base. We will consider them any mineral that contains "salts" or soluble compounds.
Ex.: Sodium Chloride (Geologic Name: Halite) and Potassium Chloride, "salt substitute" (Geologic Name: Sylvite).
Three important chemical groups in geology are built around a "radical". A radical is a group of bonded atoms that act as a single atom in a chemical reaction (think of it as a "click" or "gang"). In geology, these radicals are built around covalently bonded oxygen.
Sulfate Chemical Group
Sulfate minerals contain the radical, SO4, bonded to one or more positive "metallic" ions.
Ex.: Calcium Sulfate (Geologic Name: Gypsum)
Carbonate Chemical Group
Minerals containing the radical, CO3, bonded to one or more positive "metallic" ions are known as carbonates.
Ex.: Calcium Carbonate (Geologic Name: Calcite) and Calcium Magnesium Carbonate (Geologic Name: Dolomite)
Silicate Chemical Group
The most abundant group of minerals on the earth contain the radical, SiO4, bonded to one or more positive "metallic" ions. When bonded together, SiO4 forms a single charged silica tetrahedron. The crystal structure of individual silicate minerals varies because of the ability of the oxygens in the tetrahedron to covalently bond in one or more directions around the silica tetrahedron. Silicate crystal structures range from single tetrahedrons, long chains, double chains or rings, to sheets or complex frameworks. Because there are so many silicate minerals, they are commonly grouped into "families" with similar crystal structures and/or physical properties. For example, sheet silicates include the family of micas (Ex.: Biotite, Muscovite, Chlorite) and clays (Kaolinite). Feldspars, a family that includes orthoclase and the various plagioclases, are one of the most common silicate minerals.