Minerals

Mineral – a natural, inorganic solid with a definite chemical composition and a particular crystalline structure.

Selected minerals can be found on ESRT page 16. This page will be used for any mineral question in class or lab.

Requirements to be a mineral

• A mineral must be formed in nature – a mineral must come from the Earth’s crust and not be man-made.

• A mineral must be inorganic – not made of living matter or produced by living things. (never has lived)

• A mineral is a solid – must be solid for atoms to form characteristic patterns.

Requirements continued…

• A mineral has a definite chemical composition - always contains the same elements in the same proportions.

• A mineral has a particular crystalline structure – The atoms of a mineral are arranged in a way that forms a particular geometric shape.

Mineral Formation

Two natural processes form minerals:

1. Melted rock or magma cools to form minerals crystals.

2. Water containing dissolved minerals evaporates, leaving behind mineral crystals.

Cooling of Magma Hot liquid material called magma from the

asthenosphere moves upward through the earths crust.

As the magma moves upward it cools and the atoms in the magma “lock” into place forming crystals.

The rate at which magma cools determines the size of the crystals

• Slow cooling = large crystals• Fast cooling = small crystals

Examples: olivine, plagioclase, feldspar, quartz

Precipitation Minerals dissolved in liquids to form mixtures called

solutions. When the liquid part of the solution evaporates the

minerals are left behind.

Definition of “evaporates” – phase change when liquid water turns to water vapor by the heat energy of the Sun.

Examples of minerals formed by precipitation:Halite (NaCl)

Gypsum (CaSO4)

Mineral CompositionMost minerals share chemical similarities and

can be grouped into six major groups.

Mineral Group Composition Examples

Silicates Si + O + metals Quartz

Carbonates CO3 + metals Calcite

Oxides O + metals Magnetite

Sulfates SO4 + metals Gypsum

Sulfides S + metal Galena

Halides Cl + metal Halite

Crystal Structure

The crystal shape of minerals is determined by the arrangement of atoms that make up the mineral. We will study six main crystal shaped based on 3 or 4 lines that run through the mineral called axes.

x

y

z

Cubic - three axes of equal length intersect at 90-degree angles.

Examples: halite, galena, pyrite

Tetragonal - same as cubic, but vertical axis is longer or shorter than the others.

Examples: chalcopyrite, wulfenite

Orthorhomic - three axes of different lengths intersect at 90-degree angles.

Examples: olivine, topaz

Monoclinic - same as orthorhombic, except one axis is tilted.

Examples: mica, gypsum

Triclinic - three unequal axes intersect at titled angles to each other.

Example: plagioclase feldspar

Hexagonal - three equal horizontal axes intersect at 60-degree angles. The vertical axis is longer or shorter than others.

Example: pyromorphite

Mineral Identification

Certain physical properties are used to identify minerals. Six main properties will help use identify minerals. MUST use page 16 of the ESRT!

1. Luster

2. Streak

3. Specific gravity

4. Cleavage and fracture

5. Hardness

6. Color

Luster – How does the mineral reflect light?

• If the mineral is shiny like a metal it has a metallic luster.

• All other types of luster are grouped together, including; pearly, silky, glassy, dull, and greasy…these are called non-metallic.

Streak – What color does a mineral leave on a porcelain tile?

• The mineral is actually scratched off on the tile leaving behind the powder of the mineral.

• This is a very good test because the streak is always the same, even if color varies.

Specific Gravity – How dense is the mineral?

• The specific gravity of a mineral is the ratio of a minerals density to the density of water.

Cleavage and Fracture – How does a mineral break?

• A mineral that breaks along a flat surface of plane, has cleavage.

• A mineral that has uneven surfaces when it breaks, has fracture.

Hardness - How well does the mineral resist scratching?

• Using Mohs’ Hardness Scale we will compare the relative hardness of minerals.

Color – What color is the mineral?

• Color may be the first property you notice but it is usually the worst to identify a mineral.

• Many minerals vary greatly in color due to impurities.