Minerals are naturally occurring, inorganic solids with a definite chemical composition and a crystalline structure. Understanding their key physical and chemical properties is essential for identification and for interpreting the geological processes that formed them.
Physical Properties
1. Color
Color is often the first property noticed, but it can be misleading as many minerals occur in multiple colors due to impurities. However, some minerals have distinctive colors that aid in identification.
- Azurite: Deep blue
- Malachite: Bright green
- Sulfur: Yellow
- Hematite: Red to brown
2. Streak
Streak is the color of a mineral's powder when it is scratched across an unglazed porcelain plate. This property is often more reliable for identification than the mineral's surface color.
For example, hematite always leaves a reddish-brown streak, even though the mineral itself may appear silver, gray, or red.
3. Hardness
Hardness refers to a mineral's resistance to scratching. The Mohs Hardness Scale is used to compare the hardness of minerals:
- Talc
- Gypsum
- Calcite
- Fluorite
- Apatite
- Feldspar
- Quartz
- Topaz
- Corundum
- Diamond
4. Cleavage and Fracture
Cleavage describes how a mineral breaks along planes of weakness in its crystal structure, while fracture describes irregular or non-planar breakage.
- Perfect cleavage: Minerals like mica split easily into thin sheets
- Good cleavage: Minerals like calcite break along distinct planes
- Conchoidal fracture: Smooth, curved breaks like glass (e.g., quartz)
- Irregular fracture: Rough, random breaks (e.g., many sulfide minerals)
5. Luster
Luster describes how light reflects off a mineral's surface:
- Metallic: Shiny like metal (e.g., pyrite)
- Non-metallic: Includes vitreous (glassy), pearly, silky, and earthy varieties
6. Crystal Form
Crystal form refers to the geometric shape of a mineral's crystals, determined by its internal atomic structure.
Chemical Properties
1. Chemical Composition
Each mineral has a specific chemical composition, often expressed by a chemical formula. For example, quartz is SiO₂ (silicon dioxide).
2. Reaction to Acids
Some minerals, particularly carbonates like calcite (CaCO₃), react with dilute hydrochloric acid, producing visible bubbles of carbon dioxide gas.
3. Magnetism
Certain minerals, like magnetite (Fe₃O₄), are naturally magnetic and can be attracted to a common magnet.
Other Useful Properties
- Density: The mass per unit volume (specific gravity)
- Transparency: Whether light passes through (transparent, translucent, opaque)
- Taste: Some minerals have distinctive tastes (e.g., halite tastes salty)
- Smell: Some minerals emit odors when heated or struck
- Fluorescence: Emission of light when exposed to ultraviolet radiation