15.1 Rocks, Minerals, and the Rock Cycle
Key Takeaways
- A mineral is a naturally occurring inorganic solid with definite composition and crystal structure; a rock is an aggregate of minerals or mineraloids.
- Streak, hardness (Mohs), and the dilute-acid test on carbonates are more reliable mineral identifiers than color alone.
- Igneous rocks form from cooling magma or lava; slow cooling yields large crystals and fast cooling yields fine grains or glass.
- Sedimentary rocks form by lithification of clastic, chemical, or organic sediments; metamorphic rocks change by heat and pressure without melting.
- Rock-cycle pathways are powered by Earth's internal heat plus solar energy and gravity at Earth's surface.
Domain IV Earth and Space Science is about 26% of Praxis Middle School Science (5442). Within IV.B.1, ETS expects you to distinguish minerals from rocks, use diagnostic mineral properties, explain how the three rock types form, and trace matter and energy through the rock cycle. Teaching-scenario items often ask which property a student should measure next—or which energy source drives a particular pathway.
Minerals vs. Rocks
A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered crystal structure. Quartz (SiO₂), calcite (CaCO₃), and halite (NaCl) are minerals. A rock is a naturally occurring solid aggregate of one or more minerals (or mineraloids). Granite is a rock made of quartz, feldspar, and mica; limestone is often mostly calcite crystals cemented together.
| Feature | Mineral | Rock |
|---|---|---|
| Composition | Definite chemical formula (within a range for some groups) | Variable mixture of minerals |
| Structure | Ordered crystal lattice | Aggregate; may be crystalline, clastic, or glassy |
| Example | Olivine, pyrite, mica | Basalt, sandstone, gneiss |
| Identification focus | Properties of a single substance | Texture + mineral makeup + origin |
Classroom trap: students call anything "hard and shiny" a mineral. Counter with the definition checklist—natural, inorganic, solid, definite composition, crystal structure—and note that coal (organic) and glass (no crystal structure) fail the mineral test.
Diagnostic Mineral Properties
Color alone is unreliable: impurities tint quartz purple (amethyst), pink (rose), or milky white. Better classroom and exam tests:
| Property | What you observe | Why it helps |
|---|---|---|
| Color | Visible hue of the sample | Quick but often misleading |
| Streak | Color of the powdered mineral on an unglazed porcelain plate | More consistent than hand-sample color (hematite streaks reddish-brown even when the sample looks black or silver) |
| Hardness | Resistance to scratching; Mohs scale 1 (talc) to 10 (diamond) | Compare to known standards (fingernail ~2.5, copper penny ~3.5, glass ~5.5, steel file ~6.5) |
| Acid test | Dilute HCl fizzes on calcite (and some other carbonates) as CO₂ is released | Distinguishes calcite from similar-looking silicates |
| Luster | Metallic vs. nonmetallic (glassy, pearly, earthy) | Separates pyrite ("fool's gold") style shine from glassy quartz |
| Cleavage / fracture | Breaks along flat planes vs. irregular or conchoidal surfaces | Mica peels in sheets; quartz fractures conchoidally |
Mohs hardness is a relative scale: a mineral scratches anything softer and is scratched by anything harder. On Praxis items, students who "test hardness by color" or use streak when the stem asks for reaction with acid are selecting the wrong diagnostic tool.
Worked example — mystery white mineral
Two white samples look alike. Sample A scratches glass and does not fizz with dilute acid. Sample B does not scratch glass and fizzes vigorously. Sample A is likely quartz (hardness 7); Sample B is likely calcite (hardness 3, acid reaction). Color did not separate them; hardness and acid test did.
Igneous Rocks — From Magma and Lava
Igneous rocks form when molten rock cools and solidifies. Magma is molten rock beneath the surface; lava is molten rock that reaches the surface.
- Intrusive (plutonic) rocks cool slowly underground → large, visible crystals (phaneritic texture). Example: granite.
- Extrusive (volcanic) rocks cool quickly at or near the surface → tiny crystals or glass (aphanitic or glassy). Examples: basalt, obsidian, pumice.
Cooling rate controls crystal size: slow cooling allows ions time to migrate into large crystals; rapid cooling freezes a fine-grained or glassy texture. Composition also matters—felsic melts (richer in silica) tend toward lighter-colored rocks like granite/rhyolite; mafic melts produce darker rocks like gabbro/basalt.
Sedimentary Rocks — Pieces, Precipitates, and Organics
Sedimentary rocks form at or near Earth's surface from weathered material, chemical precipitates, or organic remains:
- Weathering breaks source rock into sediment.
- Erosion and transport move sediment (water, wind, ice, gravity).
- Deposition settles sediment in layers.
- Compaction and cementation (lithification) turn sediment into rock.
| Category | How it forms | Classroom examples |
|---|---|---|
| Clastic | Cemented fragments of prior rock | Conglomerate, sandstone, shale |
| Chemical | Minerals precipitate from solution | Rock salt (halite), some limestones |
| Organic / bioclastic | Accumulated plant/animal remains | Coal; fossiliferous limestone |
Sedimentary rocks often preserve fossils and bedding (layering)—features igneous and metamorphic rocks rarely show as clearly. Grain size in clastic rocks records energy of the depositional environment: conglomerate implies high-energy transport; shale implies quiet water.
Metamorphic Rocks — Changed by Heat and Pressure
Metamorphic rocks form when existing rock is altered by heat, pressure, and/or chemically active fluids without melting. The parent rock (protolith) recrystallizes into a new texture and sometimes new minerals.
- Foliated metamorphic rocks develop aligned mineral bands or layers from directed pressure (slate → phyllite → schist → gneiss with increasing grade).
- Nonfoliated rocks lack planar fabric; marble (from limestone) and quartzite (from sandstone) are classic examples.
If the rock melts completely, the product after cooling is igneous again—not metamorphic. That boundary is a frequent exam trap.
The Rock Cycle and Energy Flow
The rock cycle is the continuous transformation among igneous, sedimentary, and metamorphic rocks. Matter is conserved and recycled; energy drives the pathways:
| Pathway | Main energy / driving process |
|---|---|
| Melting → magma → igneous rock | Earth's internal heat (radioactive decay, residual heat) |
| Uplift and exposure | Internal heat + tectonic processes |
| Weathering, erosion, deposition | Solar energy (drives weather/climate) and gravity |
| Burial, compaction, cementation | Gravity; geothermal gradient with depth |
| Metamorphism | Internal heat + pressure from burial/tectonics |
A useful classroom model: Earth's interior heat powers melting, metamorphism, and uplift; the Sun and gravity power the surface "breakdown and rebuild" loop that makes sediment and sedimentary rock. Students who say "the rock cycle is powered only by the Sun" miss mantle heat; students who ignore sunlight miss weathering climates and the water cycle's role in erosion.
Classroom Scenario Pattern
A common Praxis teaching item: students classify granite as sedimentary "because it has different minerals stuck together." The misconception confuses aggregate with sediment. A strong response has students compare granite's interlocking crystals (cooled from melt) with sandstone's rounded grains and cement, then map both onto the rock cycle diagram—linking SEPs (developing models, arguing from evidence) to Earth materials content.
Quick Self-Check Before You Continue
- Minerals have definite composition and crystal structure; rocks are aggregates.
- Prefer streak, hardness, and acid tests over color alone.
- Cooling rate controls igneous crystal size; metamorphism changes rock without melting.
- Rock-cycle pathways are driven by Earth's internal heat plus solar energy and gravity at the surface.
A student has two white mineral samples that look identical. Sample X scratches glass and does not react with dilute hydrochloric acid. Sample Y does not scratch glass and fizzes with the acid. Which identification is best supported?
Which statement correctly distinguishes a mineral from a rock for Praxis Middle School Science (5442)?
Obsidian is glassy with no visible crystals, while granite shows large interlocking crystals. What best explains the difference?
In a rock-cycle diagram, which energy sources primarily drive melting of rock to form magma versus weathering of exposed rock into sediment?