minerals

Minerals



Contents of Entire Course

Definition of a Mineral
Atoms
Structure of Atoms
Crystal Structure
Ionic Substitution (Solid Solution)
Composition of Minerals
Properties of Minerals

 adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane University


The Earth is composed of rocks. Rocks are aggregates of minerals. Minerals are composed of atoms. In order to understand rocks, we must first have an understanding of minerals. In order to understand minerals we must have some basic understanding of atoms - what they are and how they interact with one another to form minerals.

Definition of a Mineral:

Naturally formed  it forms in nature on its own (some say without the aid of humans) Examples

Atoms

Atoms make up the chemical elements. Each chemical element has nearly identical atoms. An atom is composed of three different particles:

Each element has the same number of protons and the same number of electrons. Number of protons = Number of electrons. Isotopes  are  atoms of the same element with differing numbers of neutrons. i.e. the number of neutrons may vary within atoms of the same element. Some isotopes are unstable which results in radioactivity. Structure of Atoms

electshells.gif

Electrons orbit around the nucleus in different shells, labeled from the innermost shell as K, L, M, N, etc.  Each shell can have a certain number of electrons. The K-shell can have 2 Electrons, the L-shell, 8, the M-shell 18, N-shell 32.

#electrons = 2N2 , where N=1 for the K shell, N=2 for the L shell, N=3 for the M shell, etc.
 

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A Stable electronic configuration for an atom is one with a completely filled outer shell. Thus, atoms often loose electrons or gain electrons to obtain stable configuration. Noble gases have completely filled outer shells, so they are stable. Examples He, Ne, Ar, Kr, Xe, Rn. Others like Na, K loose an electron. This causes the charge balance to become unequal. In fact to become + (positive) charged atoms called ions. Positively charged atoms = cations. Elements like F, Cl, O gain electrons to become ( - ) charged. ( -) charged ions are called anions.

The drive to attain a stable electronic configuration in the outermost shell along with the fact that this sometimes produces oppositely charged ions, results in the binding of atoms together. When atoms become attached to one another, we say that they are bonded together.

Types of bonding:

Example: H has one electron, needs to 2 to be stable. O has 6 electrons in its outer shell, needs 2 to be stable. So, 2 H atoms bond to 1 O to form H2O, with all atoms sharing electrons, and each atom having a stable electronic configuration Packing of atoms in a crystal structure requires an orderly and repeated atomic arrangement. Such an orderly arrangement needs to fill space efficiently and keep a charge balance. Since the size of atoms depends largely on the number of electrons, atoms of different elements have different sizes. For each Na atom there is one Cl atom. Each Na is surrounded by Cl and each Cl is surrounded by Na. The charge on each Cl is -1 and the charge on each Na is +1 to give a charged balanced crystal.
The structure of minerals is often seen in the shape of crystals. The law of constancy of interfacial angles --- Angles between the same faces on crystals of the same substance are equal. This is a reflection of ordered crystal structure (See figure 2.10 in the textbook)
Crystal structure depends on the conditions under which the mineral forms. Polymorphs are minerals with the same chemical composition but different crystal structures. The conditions are such things as temperature (T) and pressure (P), because these effect ionic radii.
At high T atoms vibrate more, and thus distances between them get larger. Crystal structure changes to accommodate the larger atoms. At even higher T substances changes to liquid and eventually to gas. Liquids and gases do not have an ordered crystal structure and are not minerals.
Increase in P pushes atoms closer together. This makes for a more densely packed crystal structure. The compound Al2SiO5 has three different polymorphs that depend on the temperature and pressure at which the mineral forms.  At high P the stable form of Al2SiO5 is kyanite, at low P the stable from is andalusite, and at high T it is sillimanite. 
Carbon (C) has two different polymorphs. 

At low T and P pure carbon is the mineral graphite, (pencil lead), a very soft mineral. 

At higher T and P the stable form is diamond, the hardest natural substance known. 

In the diagram, the geothermal gradient ( how temperature varies with depth or pressure in the Earth) is superimposed on the stability fields of Carbon.  Thus we know that when we find diamond it came from someplace in the Earth where the temperature is greater than 1500oC and the pressure is higher than 50,000 atmospheres (equivalent to a depth of about 170 km). Ionic Substitution (Solid Solution)

Ionic substitution - (also called solid solution), occurs because some elements (ions) have the same size and charge, and can thus substitute for one another in a crystal structure. Note that Carbon (one of the most abundant elements in life) is not among the top 12.
Because of the limited number of elements present in the Earth's crust there are only about 3000 minerals known. Only 20 to 30 of these minerals are common. The most common minerals are those based on Si and O:  the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens covalently bonded to one silicon atom. Properties of Minerals

 Physical properties of minerals allow us to distinguish between minerals and thus identify them, as you will learn in lab.  Among the common properties used are: