Minerals - Defined
Atoms to Rocks (Figure) - Shows how the mineral is the basic building block of the geologist. In order to build minerals the atoms must join together. The process of the joining of atoms is called bonding.
There are several mechanisms through which bonding can occur but from the geologic standpoint only two are important:
Figure for NaCl. All atoms attempt to achieve the stable configuration of eight electrons in the outer most shell. To do this they can gain or lose electrons. This gain or loss causes the atoms to become charged since there is now an imbalance between positive charges (protons) and negative charges (electrons). For NaCl; chlorine gains an electron and hence a negative charge while sodium does the opposite. Ionic bonds are generally weak and many of the compounds resulting from these bonds are soluble in water.
Covalent bonds result from the sharing of electrons. See the Cl-Cl Figure. Each chlorine shares one of its outer most electrons with an adjacent chlorine atom. This sharing results in stronger bonds, particularly where multiple electrons are shared.
Crystals are built by this sharing or overlapping of electron orbitals. Since only certain arrangements minimize the mutual repulsive forces between electrons these are favored giving each crystal/mineral its unique internal geometric arrangement.
Properties of minerals that the eye can readily discern. All physical properties are to a large extent a function of the orderly nature of atoms making up the crystals and how those atoms are joined to build the crystal structure. Let’s look at some physical properties:
Classification of Minerals
A classification of minerals is a necessity if we are to talk about them since there are over three thousand different minerals. We use the anion classification system in introductory classes to pigeonhole similar minerals. This is because minerals with common anions share many common physical properties. Before discussing the classification let’s examine the abundance of elements in the earth's crust (Figure). We can expect that the most common minerals will be dominated by the most abundant elements (see below). Silicates (built from silicon and oxygen) are by far and away the most important/common minerals. Common Rock Forming Minerals table lists only the 10 most common minerals but they comprise 98% (by volume) of all minerals at the Earth’s surface.
The Common Rock Forming Minerals
Consists of a small silicon atom with a +4 charge surrounded in tetrahedral fashion by four larger oxygen atoms each having a -2 charge (Figure). Net charge on the anion group is -4. To satisfy this charge deficiency the SiO4 tetrahedra can either bond with cations (Fe, Mg, Ca, K, Na) or join with other SiO4 tetrahedra through oxygen sharing.
A) Simple silicates (Figure) Simplest structure in which each tetrahedra bonds to cations, usually Ca, Fe or Mg. Olivine is an example. Also the most dense of the silicates due to the close packing of the tetrahedra. Due to cation-tetrahedra bonds there are no stronger or weaker bonds and hence no cleavage.
B) Chain silicates (Figure) Can be either single or double chain silicates. Single chains share two basal oxygen while the double chain shares three. Two examples of this group are the pyroxenes (single chain) and amphiboles (double chain). Since the Si-O bonds are stronger than the tetrahedra-cation bonds this subgroup has fairly good cleavage in two directions.
C) Sheet silicates (Figure) Involves sharing of all basal oxygens to form a sheet of silicate tetrahedra. On top of this layer is a layer of cations, then another sheet of silicates, etc. etc. This gives the well developed basal cleavage in this group. Common sheet silicates are the micas.
D) Framework silicates - All four oxygens are shared to build up a framework of tetrahedra. Good example is quartz.
Other Anion Groups (See Slides in Class)
Carbonates - Consist of cation plus the carbonate anion (CO3-2). Important minerals calcite (calcium carbonate) and dolomite (calcium, magnesium carbonate). Calcite (limestone) the most important constituent in cement.
Oxides - Consist of oxygen plus a cation, often Fe, Ti, Al, Cu, or Cr. Important oxides include hematite (Fe oxide) magnetite (Fe oxide). Major source of world’s iron, aluminum and chromium.
Sulfides - Sulfur plus a metallic cation. Most of our important ore minerals are in this group. Includes galena (PbS), pyrite (FeS2) and numerous copper sulfides. Another important group of ore minerals.
a) Sulfates (S04) - Gypsum used for drywall
b) Halides (Cl,F,Br) - Rock salt
c) Phosphates (P04) - Apatite for fertilizer