Minerals & crystals

I. Minerals are to rocks what vegetables are to a salad. If you know what vegetables are in your salad, you can tell what kind of salad you have. Likewise, if you can identify the minerals in your rock, you can name it. Minerals are the fundamental unit for understanding rocks.

II. For purposes of geology, a  mineral is defined a

A. Naturally occuring -- synthetic diamonds or cubic zirconium technically are not naturally occuring

B. Inorganic (means that coal isn't a mineral because coal comes from organic material) C. Solid

D. Has a definite chemical composition (a predictable formula)

E. Crystalline  (means it has an ordered atomic arrangement)

III. Chemistry

A. Ions

1. Ions  are atoms that have a charge

2. The charge is determined by adding up the positive and negative charges (negative charges are equivalent to negative numbers for purposes of calculating the charge).

3. A charge occurs because, although a neutrally charged atom will have an equal number of protons and electrons, atoms normally like to have its electrons arranged in orbits of 2 or 4 electrons. Thus an atom will either get rid of the excess charges, or it will capture extra electrons to make a full orbit.

B. Chemistry of minerals is determined by the charge of the ions that form its crystal structure. The overhead example demonstrated that the sodium ion had a net charge of +1 and the chlorine ion a net charge of -1. Ions like to combine so as to make a neutral charge (0). 

IV. Silicate minerals are the most common family of minerals on earth. These minerals are all composed of Si (silicon) and O (oxygen).

A. Silicon and oxygen tend to combine so as to form a  silica tetrahedron. See pg. 34 of Plummer & McGeary

1. A tetrahedron is a geometric shape having four triangular-shaped sides.

2. A single silica tetrahedron has 4 oxygens and one silicon atom and has a net charge of -2 (Si⁺⁴ and 4 O^-2).

B. Silica tetrahedra combine in different ways to produce different minerals.

1. Isolated tetrahedra combine with magnesium and iron to make  olivine.

2. Tetrahedra that share two of its oxygens with neighbors form a chain silicate mineral called pyroxene.

3. When two chains combine to form double chain silicates, they form minerals called amphiboles.

4. Tetrahedra that share 3 oxygens as sheet silicates form minerals called  micas.

5. Feldspars and  quartz are made when all four oxygens are shared with neighboring tetrahedra.

V. Carbonates are another common family of minerals whose most common member is calcite. It has a chemical formula CaCO³.

VI. How do we identify minerals?

A. Hardness --  Moh's hardness scale runs from one to ten in order of increasing hardness. Each number on the scale is represented by a reference mineral. For instance, calcite has a hardness of 3, quartz is 7, and diamond is 10.

B. Crystal structure

1. Crystal form -- this is the shape that a crystal will take when it grows unimpeded.

2. Cleavage -- this is the shape a crystal has when it is broken up. Cleavage refers to planes of weakness in a crystal structure along which a crystal tends to break.

C. Color -- this works for some minerals, but not all. Minerals with Fe and Mg tend to be darker in color (green, brown or black).

D. Streak is a variation of color. Specifically it is the color of a mineral in its powdered form.

E. Luster -- describes the way light is reflected off of a mineral. Terms like metallic, glassy, resinous, and pearly are often used.

F. Other properties -- taste, density, magnetism

VII. The rock cycle describes the relationship between the three different types of rocks

A. Igneous  rocks form from magma or molten rock

B. Sedimentary  rock forms from weathering and consolidation of sediment by cement.

C. Metamorphic rocks occur under conditions of high pressure and temperature (but not hot enough to melt it).