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Mineral identification is the foundation of Physical Geology—you can't interpret rock formation, metamorphic processes, or economic geology without first knowing what minerals you're looking at. The properties covered here aren't random physical traits; they're direct expressions of a mineral's crystal structure and chemical bonding. When you test hardness, you're measuring bond strength. When you observe cleavage, you're seeing planes of atomic weakness. Every property tells a story about how atoms are arranged.
On exams, you're being tested on your ability to systematically identify minerals and explain why properties vary. Don't just memorize that diamond is hard and talc is soft—understand that diamond's hardness comes from its three-dimensional covalent bonding, while talc's softness reflects weak bonds between silicate sheets. Know which properties are reliable, which are misleading, and how to combine multiple tests for accurate identification.
The internal arrangement of atoms in a mineral determines its external form and how it breaks. These properties reveal the geometry of atomic bonding—the invisible architecture made visible.
Compare: Cleavage vs. Fracture—both describe how minerals break, but cleavage follows atomic planes (smooth, predictable) while fracture doesn't (irregular, curved). If an exam shows a broken mineral surface, ask: Is it flat and reflective (cleavage) or curved/rough (fracture)?
How tightly atoms are held together determines a mineral's resistance to physical stress. These properties are quantifiable and highly reliable for identification.
Compare: Hardness vs. Specific Gravity—both relate to atomic properties, but hardness measures bond strength (resistance to scratching) while specific gravity measures atomic mass and packing (density). A mineral can be soft but dense (galena) or hard but light (topaz).
How minerals interact with light depends on their surface texture, chemical composition, and electronic structure. These visual properties are your first observations but require careful interpretation.
Compare: Color vs. Streak—color is what you see on the surface (variable, affected by impurities and weathering), while streak is the powder color (consistent, more reliable). Hematite is the classic example: it can appear silver, black, or red, but the streak is always red-brown.
Some minerals exhibit unique physical or chemical behaviors that provide definitive identification—these are your "trump card" tests when other properties are ambiguous.
Compare: Magnetism vs. Acid Reaction—both are "special" tests for specific mineral groups. Magnetism identifies iron-bearing minerals (magnetite, pyrrhotite), while acid reaction identifies carbonates (calcite, dolomite). Neither works for silicates, which require other properties.
| Concept | Best Examples |
|---|---|
| Crystal structure → external form | Crystal form, Cleavage, Fracture |
| Bond strength properties | Hardness, Specific gravity |
| Light interaction properties | Luster, Color, Streak |
| Chemical composition tests | Acid reaction, Magnetism |
| Most reliable for ID | Streak, Hardness, Cleavage, Specific gravity |
| Least reliable for ID | Color (too variable) |
| Metallic mineral diagnostics | Luster, Streak, Specific gravity |
| Carbonate diagnostics | Acid reaction, Rhombohedral cleavage |
A mineral breaks into flat, reflective surfaces at 90° angles. Another mineral of similar composition breaks with curved, shell-like surfaces. What properties are you observing, and what does each tell you about atomic structure?
You find two yellow minerals in the field. One feels noticeably heavy; the other feels light. One leaves a black streak; the other leaves a yellow streak. Which properties would you use to distinguish them, and why is color alone insufficient?
Compare and contrast hardness and cleavage—both relate to atomic bonding, but what specifically does each property measure, and how might a mineral be hard but have perfect cleavage?
A student identifies a mineral as calcite based on its rhombohedral cleavage, but their lab partner thinks it's dolomite. What single test would definitively distinguish between them, and what result would you expect for each?
Rank the following properties from most to least reliable for mineral identification: color, streak, hardness, luster. Justify your ranking with specific examples of how unreliable properties can mislead you.