Notes
Minerals
- A mineral is natural, inorganic, solid, has a definite chemical composition and a crystal structure.
- Identify minerals with streak, luster, hardness (Mohs 1-10), cleavage or fracture, density and special tests (acid, magnetism).
- Density $\displaystyle = \frac{\text{mass}}{\text{volume}}$. Silicates (oxygen and silicon) are the most common minerals.
Three rock types
- Igneous: cooled magma or lava. Slow cooling gives large crystals (intrusive); fast cooling gives small crystals or glass (extrusive).
- Sedimentary: sediment lithified by compaction and cementation. Layers and fossils are common.
- Metamorphic: changed by heat and pressure without melting. Foliated (banded) or nonfoliated.
The rock cycle
Rocks change among the three types through weathering, erosion, lithification, metamorphism, melting and cooling. Any rock can follow many paths.
Topics in this unit
Each topic has its own short notes and practice.
Practice
$120 \div 40 = 3.0$ g/cm³.
Minerals must be solid and crystalline.
Fine grain and bubbles indicate rapid cooling extrusive rock like basalt.
That describes lithification of sediment.
Rock changes in the solid state due to heat and pressure.
Any rock can melt and form igneous rock.
Free response
Write your answer first, then open the scoring guide.
Free-response question
A geologist finds a gray mineral that scratches glass (hardness about 5.5) but is scratched by quartz (hardness 7). A 210 g piece has a volume of 42 cm³. She also finds a rock with large interlocking crystals near the same site.
(a) Calculate the mineral's density.
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$210 \div 42 = 5.0 \text{ g/cm}^3$.
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(b) Explain why hardness of 5.5 to 7 can narrow down the identity but is not enough alone, and name one more test.
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Many minerals fall in that hardness range, so other properties must also be tested. Examples: streak, luster, cleavage, density or a magnet test.
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(c) The rock with large crystals is granite. Describe how it formed and how it could become a metamorphic rock.
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Granite formed from magma that cooled slowly underground, giving large crystals. Heat and pressure could change it into gneiss without melting.
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