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Grade 11-12 (age 16-18)

high school Minerals and Rocks

Rocks at the surface are classified by how they formed into igneous, sedimentary, and metamorphic rocks, but even a rock that has already formed turns into another kind over long stretches of time. Magma cools into igneous rock; that rock breaks down, piles up, and hardens into sedimentary rock, turns metamorphic under heat and pressure underground, or melts back into magma, and this whole loop is called the rock cycle. In igneous rock, crystal size depends on how fast the magma cooled, and the minerals that make up rocks can be told apart by properties such as hardness, streak, cleavage, and luster. Choosing a process in the diagram brightens that arrow and the rocks it starts and ends at, and the text below describes what happens in that step.

Three ways a rock can form

🪨 The three kinds of rock
①Igneous rock forms when magma cools and solidifies; granite and basalt are typical
②Sedimentary rock forms when broken grains, dissolved substances, or the remains of living things pile up and harden, and it may show layers and fossils
③Metamorphic rock is existing rock whose minerals and texture changed under heat and pressure without melting
④The three kinds change into one another over long periods, so none of them is a final stage

The rock cycle

cooling
💡 The five steps in the diagram
①Cooling and solidification: magma cools into igneous rock
②Weathering and erosion: rock at the surface breaks down and wears away into sediment
③Compaction and cementation: buried sediment is squeezed and its grains glued together into sedimentary rock
④Heat and pressure: sedimentary rock buried deep becomes metamorphic rock
⑤Melting: metamorphic rock melts back into magma. As the dashed lines show, there are also shortcuts, such as igneous rock being metamorphosed directly or metamorphic rock weathering back into sediment

How rocks are classified

Rock classification

Subtypes of the three kinds of rock

Properties of minerals

Definition of mineral
natural + inorganic + solid + crystalline + definite chemical composition
All five conditions must be met, so a pearl, made by a living thing, and obsidian, which has no crystal structure, are not minerals.

Telling minerals apart

Physical properties of minerals
Hardness
scratch two minerals together and the one that gets scratched is softer; talc 1, quartz 7, diamond 10
Mohs scale 1 to 10
Streak
can differ from the surface color (pyrite looks yellow but leaves a black streak)
color of the powder on a streak plate
Cleavage and fracture
mica cleaves in one direction and calcite in three, while quartz shows fracture
splitting along flat planes is cleavage
Luster
how the surface reflects light
metallic / nonmetallic

Worked Examples and Exam Practice

Example 1
What is an igneous rock called when magma cools slowly deep underground, forming large crystals, and what is a typical example?
1
Magma cooling slowly deep underground grows large crystals → plutonic (intrusive) rock.
2
A typical plutonic rock is granite. Cooling fast at the surface instead gives fine-grained volcanic rock (basalt).
▸ Plutonic rock (e.g., granite)
The longer crystals have to grow, the bigger they get, so slowly cooled plutonic rock has large crystals and quickly cooled volcanic rock has small ones.
Example 2
On the Mohs scale, diamond is 10 and quartz is 7. If you scratch quartz with diamond, which one gets scratched?
1
The Mohs scale is a relative hardness ranking based on which mineral scratches which.
2
A harder mineral scratches a softer one, so diamond (10) scratches quartz (7).
▸ The quartz gets scratched
The Mohs scale only ranks minerals in order, so the gap between 7 and 10 is not the same size as the gap between 1 and 4.
CSAT-style problem
Which statement about minerals and rocks is correct?
①Plutonic rock cools fast at the surface, giving small crystals
②Metamorphic rock forms under heat and pressure
③Coal and limestone are igneous rocks
④The higher the Mohs hardness, the softer the mineral
⑤Sedimentary rock forms from solidified magma
▸ ② Metamorphic rock forms under heat and pressure
1
Metamorphic rock is existing rock whose minerals and texture have changed under high heat and pressure (gneiss, marble).
2
Plutonic rock cools slowly with large crystals (①), coal and limestone are sedimentary (③), higher Mohs means harder (④), and solidified magma gives igneous rock (⑤).

Summary

Minerals & Rocks core
magma → igneous rock → sediment → sedimentary rock → metamorphic rock → magma (with shortcuts)
Igneous rocks are sorted by cooling rate, sedimentary rocks by what piled up, and metamorphic rocks by the heat and pressure they received; minerals are told apart by hardness, streak, cleavage, and luster.
🎯 Exam Points
①Plutonic rock cools slowly and has large crystals; volcanic rock cools quickly and has small ones
②Sedimentary rock has bedding, may contain fossils, and is divided into clastic, chemical, and organic
③Regional metamorphic rock develops foliation because of pressure (schist, gneiss)
④In Bowen's reaction series, olivine crystallizes first and quartz last as magma cools
⑤Mohs hardness: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10
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