Minerals

The Earth is composed of rocks. Rocks are aggregates of minerals. Minerals are composed of atoms

Definition of a Mineral:

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


Solid ( it cannot be a liquid or a gas)


With a definite chemical composition (every time we see the same mineral it has the same chemical composition that can be expressed by a chemical formula).


and a characteristic crystalline structure (atoms are arranged within the mineral in a specific ordered manner).
Examples

Glass - can be naturally formed (volcanic glass called obsidian), is a solid, its chemical composition, however, is not always the same, and it does not have a crystalline structure. Thus, glass is not a mineral.


Ice - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula H2O, and does have a definite crystalline structure when solid. Thus, ice is a mineral, but liquid water is not (since it is not solid).


Halite (salt) - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula NaCl, and does have a definite crystalline structure. Thus halite is a mineral.

Composition of Minerals

The variety of minerals we see depend on the chemical elements available to form them. In the Earth's crust the most abundant elements are as follows:

O, Oxygen 45.2% by weight

Si, Silicon 27.2%

Al, Aluminum 8.0%

Fe, Iron 5.8%

Ca, Calcium 5.1%

Mg, Magnesium 2.8%

Na, Sodium 2.3%

K, Potassium 1.7%

Ti ,Titanium 0.9%

H, Hydrogen 0.14%

Mn, Manganese 0.1%

P, Phosphorous 0.1%
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:

Habit - shape

Color

Streak (color of fine powder of the mineral)

Luster -- metallic, vitreous, pearly, resinous (reflection of light)

Cleavage (planes along which the mineral breaks easily)

Density (mass/volume)

Hardness: based on Mohs hardness scale as follows:
Talc

gypsum (fingernail)

calcite (penny)

fluorite

apatite (knife blade)

orthoclase (glass)

quartz

topaz

corundum

Diamond

Formation of Minerals

Minerals are formed in nature by a variety of processes. Among them are:

Crystallization from melt (igneous rocks)


Precipitation from water (chemical sedimentary rocks, hydrothermal ore deposits)


Biological activity (biochemical sedimentary rocks)


Change to more stable state - (the processes of weathering, metamorphism, and diagenesis).


Precipitation from vapor. (not common, but sometimes does occur around volcanic vents)
Since each process leads to different minerals and different mineral polymorphs, we can identify the process by which minerals form in nature. Each process has specific temperature and pressure conditions that can be determined from laboratory experiments. Example: graphite and diamond, as shown previously.

Rocks - Mixtures of Minerals

Mixtures or aggregates of minerals are called rocks. There are three basic kinds of rocks, each type is determined by the process by which the rock forms.

Igneous Rocks - form by solidification and crystallization from liquid rock, called magma.


Sedimentary Rocks - form by sedimentation of mineral and other rock fragments from water, wind, or ice and can also form by chemical precipitation from water.


Metamorphic Rocks - form as a result of increasing the pressure and/or temperature on a previously existing rock to form a new rock.

Each of these rock forming processes results in distinctive mineral assemblages and textures in the resulting rock. Thus, the different mineral assemblages and textures give us clues to how the rock formed.