Earth and Space Science / Unit 2: Minerals and Rocks
2.2 Identifying Minerals
Notes
Minerals are identified by physical properties, not color alone.
- Luster: how it reflects light (metallic or nonmetallic).
- Streak: the color of the powder left on an unglazed porcelain plate. It is more reliable than surface color.
- Hardness: resistance to scratching, measured on the Mohs scale from 1 (talc) to 10 (diamond). Quartz is 7, calcite 3, a fingernail about 2.5, a steel nail about 5.5.
- Cleavage is breaking along flat planes; fracture is breaking along irregular surfaces.
- Density $\displaystyle = \frac{\text{mass}}{\text{volume}}$. Example: a 150 g sample with volume 30 cm³ has density $150 \div 30 = 5.0 \text{ g/cm}^3$.
- Special tests: calcite fizzes in weak acid; magnetite is magnetic; halite tastes salty.
Practice
A mineral scratches calcite (3) but is scratched by quartz (7). Its hardness could be:
Its hardness is between 3 and 7.
A mineral sample has a mass of 90 g and a volume of 25 cm³. Its density is:
$90 \div 25 = 3.6$ g/cm³.
Why is streak more useful than color for identifying a mineral?
Impurities change a mineral's color, but streak is consistent.
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