What Is a Mineral?
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered atomic structure.
When studying mineral formation, consider the environmental conditions: temperature, pressure, and the presence of water or other chemicals. These factors determine which minerals form and their properties.
Physical Properties of Minerals
1. Color
- The most noticeable property but often unreliable.
- Example: Quartz can be clear, pink (rose quartz), or purple (amethyst).
Don't rely solely on color for identification. Many minerals, like calcite and halite, can appear in similar colors.
2. Luster
- Describes how light reflects from a mineral's surface.
- Metallic: Shiny like metal (e.g., pyrite).
- Nonmetallic: Glassy, pearly, or earthy (e.g., quartz).
3. Streak
- The color of a mineral's powder when rubbed on a streak plate.
- Example: Pyrite has a brassy color but leaves a greenish-black streak.
4. Hardness
- A mineral's resistance to scratching, measured by Mohs scale (1 to 10).
- Example: Talc (1) is the softest; diamond (10) is the hardest.
Cleavage occurs along planes of weakness in a mineral's atomic structure, while fracture happens when there are no such planes.
6. Specific Gravity
- The density of a mineral compared to water.
- Example: Galena has a specific gravity of 7.5, meaning it is 7.5 times denser than water.
7. Chemical Tests
- Acid Test: Calcite reacts with hydrochloric acid, producing bubbles of carbon dioxide.
- Taste Test: Halite tastes salty (use with caution).
Avoid tasting minerals unless instructed by a teacher, as some minerals can be toxic.
8. Special Properties
- Magnetism: Magnetite is attracted to magnets.
- Luminescence: Fluorite glows under ultraviolet light.
- Double Refraction: Iceland spar calcite creates a double image when viewed through it.
Silicates make up over 90% of Earth's crust due to the abundance of silicon and oxygen.
2. Non-silicates
- Sulfides: Contain sulfur and a metal (e.g., pyrite, \$FeS_2\$).
- Oxides: Contain oxygen and a metal (e.g., hematite, \$Fe_2O_3\$).
- Carbonates: Contain the carbonate ion (\$CO_3^{2-}\$) (e.g., calcite, \$CaCO_3\$).
- Sulfates: Contain the sulfate ion (\$SO_4^{2-}\$) (e.g., gypsum, \$CaSO_4 \cdot 2H_2O\$).
- Halides: Contain a halogen element (e.g., halite, \$NaCl\$).
- Native Elements: Pure elements (e.g., gold, silver).
Think of Earth's mineral resources like a savings account. Once depleted, they cannot be quickly replenished, emphasizing the need for careful management and conservation.
