Luster: This describes how light reflects off the surface of a mineral.
* Metallic: Looks like polished metal (e.g., pyrite, galena).
* Non-metallic:
* Vitreous: Glassy (e.g., quartz).
* Dull: No shine (e.g., kaolinite).
* Resinous: Like resin (e.g., amber).
* Silky: Like silk (e.g., asbestos).
* Pearly: Like a pearl (e.g., talc).
* Earthy: Like soil (e.g., hematite).
Cleavage: This refers to how a mineral breaks along specific planes of weakness.
* Perfect: Breaks cleanly and evenly (e.g., mica).
* Good: Breaks fairly evenly but with some unevenness (e.g., feldspar).
* Poor: Breaks unevenly with no clear pattern (e.g., quartz).
* Fracture: Breaks irregularly, not along planes (e.g., quartz).
Hardness: This measures a mineral's resistance to scratching. It's determined using the Mohs Hardness Scale, where 1 is the softest (talc) and 10 is the hardest (diamond).
* Soft: Scratched by a fingernail (e.g., talc, gypsum).
* Medium: Scratched by a copper coin (e.g., calcite).
* Hard: Scratched by a steel knife (e.g., quartz).
* Very Hard: Not scratched by a steel knife (e.g., topaz, corundum).
Color: While color is a helpful initial indicator, it can be unreliable for identifying minerals.
* Streak: The color of the mineral's powder when rubbed against a streak plate (a piece of unglazed porcelain) is more reliable than its overall color.
Rocks vs Minerals:
* Minerals: Naturally occurring, solid, inorganic substances with a definite chemical composition and crystalline structure.
* Rocks: Solid aggregates of one or more minerals, or even non-mineral materials like glass (e.g., obsidian).
Identifying Rocks:
While these mineral properties are useful, identifying rocks often involves considering other characteristics like:
* Texture: The size, shape, and arrangement of the mineral grains (e.g., coarse-grained, fine-grained, porphyritic).
* Composition: The types of minerals present (e.g., granite contains quartz, feldspar, and mica).
* Origin: How the rock formed (igneous, sedimentary, or metamorphic).
In summary:
Luster, cleavage, hardness, and color are important properties for identifying minerals, which in turn help us understand the composition and origin of rocks.
