A Rock is a naturally formed, consolidated material composed of grains of one or more minerals. Two examples of rocks are :

Granite is a rock composed of grains of several minerals including feldspar and quartz.

Rock Salt is a rock composed of grains from only one mineral (Halite).

Chemical composition of the earth's crust is as follows:

Element Atomic Symbol Percentage by Weight

Oxygen O 46.6

Silicon Si 27.7

Aluminum Al 8.1

Iron Fe 5.0

Calcium Ca 3.6

Sodium Na 2.8

Potassium K 2.6

Magnesium Mg 2.1

All others 1.5

A Mineral is composed of atoms arranged in a very orderly, three-dimensional structure which is to say it is crystalline. For example, the formula for halite is NaCl which means that it is composed of equal numbers of sodium (Na) and chlorine (Cl). In the crystalline lattice each sodium atom is surrounded by six chlorine atoms and each chlorine atom is surrounded by six sodium atoms. Most rocks are crystalline. To be a mineral in a geologic sense a substance must satisfy five conditions:

1. It must be a crystalline solid.

2. It must occur naturally.

3. It must be inorganic

4. It must have a definite chemical composition.

5. It must possess characteristic physical properties including the following:

Color (i.e. black, white, pink, yellow, red, purple, etc.)

Streak - the color of scraping the edge of a mineral across an unglazed porcelain plate.

Luster - quality and intensity of light that is reflected from the surface of the substance.

Hardness - scratchability as indicated by Mohs' hardness scale.

External crystal form - A mineral is a set of faces that have a definite geometric relationship.

Cleavage - tendency to split apart along certain preferred directions (quartz olivine, garnet have no cleavage).

Fracture - some minerals show a conchoidal fracture.

Specific gravity - Heaviness or density of the mineral. Specific gravity is the ratio of the mass of a substance to the mass of an equal volume of water. (Quartz = 2.65; Gold = 19.3)

Magnetic - Magnetite is an iron oxide which is magnetic.

Double refraction - Light splits into two components when it enters some crystalline materials such as calcite.

Polymorphs - Those compounds which are known to form two or more different kinds of crystal structures. (e.g. Diamond and Graphite; Quartz, Cristobalite, and Tridymite).

Mineraloids - Those compounds which lack crystal structure or definite composition (e.g. Opal)

Important Minerals of the Earth's Crust

Silicates are the most important rock forming mineral. All are substances which contain silicon are silicates. Because silicon is so abundant in the crust we would expect that silicate minerals would be a major component of most rocks. These minerals will be discussed in a separate lecture. They include:

Quartz, feldspar group, mica group, amphibole group, pyroxene group, olivine, garnet group.

NONSILICATES:

Carbonates:

Calcite - CaCO3 - A carbonate which is deposited by chemical precipitation as well as deposited by some animals in their shells. Coral reefs are predominantly calcite.

Dolomite - CaMg (CO3)2 - A carbonate often deposited as calcite and later converted with the addition of Mg.

Native Elements:

Diamond - C - A native element of great value because of its hardness. Found in kimberlite deposits in the Republics of Zaire and South Africa and USSR.

Gold - Au - Valuable as a monetary standard.

Oxides:

Hematite - Fe203 - This is the most important ore mineral for iron.

Magnetite - Fe203 - This magnetic mineral is one of the most widespread oxide minerals in the earth. It is a minor accessory in igneous rocks.

Cassiterite - SnO2 - The main ore mineral of tin.

Uraninite - U3O8 - The main ore mineral of uranium.

Hydroxides:

Boehmite - AlO(OH) - This mineral is an important component of bauxite which is an important ore mineral of aluminum.

Sulfides:

Sphalerite - (Zn,Fe)S - Provides the bulk of the world's zinc.

Chalcopyrite - CuFeS2 - "Fools Gold" This mineral is the primary copper mineral in porphyry-copper deposits.

Galena - PbS - This is the most important lead mineral in the earth's crust.

Sulphates:

Gypsum - CaSO4.2H2O - Deposited before halite as part of an evaporite deposited when large bodies of water evaporate without draining. An example is the Great Salt Lake of Utah.

Anhydrite - CaSO4 - The product of gypsum when it has dewatered.

Phosphates:

Apatite - Ca5(PO4)3(F,OH) - Common in igneous and sedimentary rocks and is the main source of the phosphorus used in making Phosphate fertilizers.

Halides:

Halite - NaCl - Deposited after gypsum as part of an evaporite deposited when large bodies of water evaporate without draining. An example is the Great Salt Lake of Utah.