Becca Hedges
Geo 3210
Dr. Kaiser
3rd Oct. 2016
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Exam 1 Essay
A mineral is an inorganic solid, formed by natural geologic processes, and has a definite crystalline structure.
Quantum Numbers
The crystalline structure is the most complex requirement of a mineral. To better help us understand how this works we use chemistry. Each element has a specific set of quantum numbers. These are what controls how an element behaves. The principal (n) are each shell (k-q). This is how close an electron is to the nucleus. The last shell is where bonding will occur. The angular (l) subshells control the shape, (s,p,d,f,g). The magnetic (ml) sub orbitals have magnetic properties. Hund’s Rule tell us that the spin (½) must fill each orbital a + ½ first, can then fill the orbital with a - ½ as the number of electrons allow.
Bonding
Chemical bonds can form thru Valence related bonding (Ionic, Covalent, and Metallic) or Non-Valence related bonding (Hydrogen or Vander Walls). Electrons are very important in minerals ionic bonding has. Each one has an electron donor and an acceptor. Cations and Anions like NaCl for this type of bond. Covalent bonds occur when the outer electrons are being shared like Cl2. Nonmetals have some of the strongest bonds and are high in electronegativity. Covalent bonds are also where hybridizations of orbitals occur. Metallic bonding are some of the weakest types and have low electronegativity. They are malleable, deform easily, are conductive, and hold electrical/thermal properties. Pauling’s five rules also help describe how bonding occurs. Rule 1- cations will bond with as many anions it can based off of their relative sizes. Rule 2- based off of the coordination numbers the will stack in one of two ways. A uniform manner and have a strong bond, or a non-uniform which can be quite complicated structures. Rule 3- nature prefers simplicity. Sharing of the corners is the best fit. Rule 4- Elements do not want to share any more than they have too. Rule 5- Cations will systematically arrange themselves into no more than four coordination polyhedral.
Substitutions
Substitutions can also play are large role in mineral making. The three compositional variations are size, charges, and temperature. There are four different ways in which one would occur. The first is simple substitution. Cations with the same charges and size can act like the other. For example, olivine (Fe2+, Mg2+)SiO4, the Fe and Mg can substitute for one another making either Forsterite or Fayalite . The second is called Coupled substitution. This type maintains balance of the charges by increasing the charge of another. For example Plagioclase can be Ca2+ Al2 3+ Si2 4+ O8 (Anorthite) or Na+Al3+ Si3 4+ O8 (Albite). This type of substitution can have different sizes of elements at high temperatures. The third is Omission substitution. This way still allows for cations of different sizes but instead of all of the spaces being filled, some are left empty. An ion of a smaller charge will be replaced but the higher charge is what is left empty. For example Pyrrhotite, Fe2+ Fe3+ have variable amounts. The fourth is interstitial substitution. This is a variation of a coupled substitution in which ions are placed into sites normally left empty while maintaining the charge balance. Beryl is an example of interstitial substitution.
Crystal Lattice and Structure
A crystal lattice is a 3-D network that is repeating at fixed lattice points across planes. The unit cell is the smallest unit which is repeated. Crystal facies grow in orientations along the lattice points in one of two ways. The law of Hauy which says the facies make simple intercepts at the axis. The law of Bravais says common facies are parallel to the planes and have a high density of atoms. The number of possible lattices are limited. Crystal structures are ions packed together in an efficient manner. There are six different systems. Isometric axis are of equal length and are perpendicular, for example fluorite. Hexagonal has two axis that are the same length and are perpendicular to each other, for example quartz. Tetragonal has one axis that is a different length but all are perpendicular, for example zircon. Orthorhombic has three axis that are different lengths but are all perpendicular, for example olivine. Monoclinic z axis is perpendicular with varying lengths. One side will be at an angle, for example gypsum. Triclinic has no symmetry. All three axis are different lengths and all three have different angles, for example kyanite.
Mineral Nucleation and Growth
Crystal nucleation is the formation of a nuclei in one of two ways. A homogeneous nucleation needs the orientations of the atoms and ions to be arranged so they can bond. Atoms and ions come together and grow. Most ions want to bond and non-bonded are unstable. This also must happen fast to overcome the energy required. The more the surface area there is the more likely they will continue growth. A heterogeneous nucleation occurs new minerals sneak in on a similar mineral and begin growing with it. The law of Bravais is also applied here. Crystals don’t want to grow in single layers. The will stack in pieces. All of this can be influenced by temperature and pressure conditions.
We know all of this because of our knowledge with chemistry, and work done by Pauling, Bravais, and others. We also use phase diagrams to help understand where these events are more likely to occur.