Rocks & Minerals

What Are Rocks and Minerals?

Minerals are naturally occurring, inorganic crystalline solids with a defined chemical composition. Examples include quartz (SiO2), calcite (CaCO3), feldspar, and mica.

Rocks are mixtures of one or more minerals. They are classified into three main types based on how they formed: igneous, sedimentary, and metamorphic.

Igneous Rocks

Igneous rocks form when magma (molten rock underground) or lava (magma that reaches the surface) cools and solidifies.

The rate of cooling determines crystal size:

• Slow cooling (underground) → large crystals (more time for crystals to grow)
• Fast cooling (at the surface) → small crystals or glassy texture

Intrusive Igneous Rocks

Cool slowly deep underground in large bodies called batholiths or plutons. Form large, visible crystals. Examples:

• Granite — contains large crystals of quartz, feldspar, and mica; hard and resistant

Extrusive Igneous Rocks

Cool rapidly at the surface after volcanic eruption. Form small crystals. Examples:

• Basalt — fine-grained; forms ocean floor; makes up most of Earth's crust
• Obsidian — cooled so fast it formed volcanic glass (no crystals)
• Pumice — full of gas bubbles; so light it floats on water

Sedimentary Rocks

Sedimentary rocks form from the accumulation and compaction of sediment (rock fragments, mineral grains, shell fragments, organic material) over millions of years.

Formation process:

1. Weathering and erosion — existing rocks are broken down into fragments
2. Transportation — fragments carried by water, wind, or ice
3. Deposition — fragments settle in layers (strata) in lakes, rivers, or the sea
4. Compaction — layers are buried and compressed by the weight above
5. Cementation — minerals dissolved in water crystallise between particles, binding them together

Examples and their origins:

• Sandstone — compacted sand grains; often found in desert or river environments
• Limestone — formed from shells and skeletons of marine organisms (mainly calcite)
• Chalk — soft white limestone; formed from tiny marine organisms (coccoliths)
• Mudstone/shale — compacted clay particles

Sedimentary rocks often contain fossils because organisms can be buried quickly in sediment before they decay. They also show visible layers (strata).

Metamorphic Rocks

Metamorphic rocks form when existing rocks are subjected to intense heat and/or pressure (but not melted — melting produces igneous rock). The minerals recrystallise into new arrangements.

This happens at:

• Tectonic plate boundaries (subduction zones, collision zones)
• Contact zones with magma intrusions (contact metamorphism)

Common metamorphic transformations:

• Limestone → Marble (calcite crystals grow larger; used in sculpture)
• Mudstone/shale → Slate (thin flat layers; used in roofing)
• Sandstone → Quartzite (very hard; quartz grains fuse together)
• Granite → Gneiss (banded appearance; high-grade metamorphism)

The Rock Cycle

All three rock types can be transformed into each other over geological time:

• Any rock + weathering/erosion → sediment → sedimentary rock
• Any rock + heat and pressure → metamorphic rock
• Any rock + melting → magma → cooling → igneous rock
• Metamorphic or igneous rocks can be uplifted to the surface, then weathered to form sediment

The rock cycle has no beginning or end — it is a continuous, slow process driven by heat from Earth's interior and energy from the Sun.

Minerals and Their Properties

Minerals are identified by their physical properties:

Property	Description	Example
Hardness	Resistance to scratching (Mohs scale 1–10)	Diamond = 10; talc = 1
Colour	Can be diagnostic but variable	Malachite = green
Lustre	How light reflects (metallic, glassy, waxy)	Pyrite = metallic
Crystal shape (habit)	Characteristic form of crystals	Halite = cubic
Cleavage	How mineral splits along flat planes	Mica splits into sheets
Streak	Colour of mineral's powder on white tile	Haematite = red-brown
Density	Mass per unit volume	Gold = 19.3 g/cm cubed

The Mohs hardness scale runs from 1 (softest — talc, scratched by a fingernail) to 10 (hardest — diamond, scratches everything else).

Weathering

Weathering is the breakdown of rocks in place (without transport). Three types:

Physical (mechanical) weathering:

• Freeze-thaw (frost action): water enters cracks, freezes, expands (by about 9%), widens cracks
• Thermal expansion: rock expands and contracts with temperature changes
• Abrasion: rocks grind against each other during transport

Chemical weathering:

• Carbonation: slightly acidic rainwater (carbonic acid) dissolves limestone — forms caves, stalactites, and stalagmites
• Oxidation: iron-rich minerals react with oxygen, forming rust-coloured iron oxides
• Hydrolysis: minerals react with water and break down

Biological weathering:

• Plant roots grow into cracks and widen them
• Lichens produce acids that dissolve rock surfaces

Key Terms

• Igneous — rock formed from cooling magma or lava
• Sedimentary — rock formed from compacted sediment layers
• Metamorphic — rock transformed by heat and/or pressure
• Mineral — naturally occurring inorganic solid with defined chemical composition
• Rock cycle — continuous process converting rocks from one type to another
• Weathering — breakdown of rocks in place
• Fossil — preserved remains or traces of ancient organisms in sedimentary rock

Common Mistakes

• Confusing marble (metamorphic, from limestone) with limestone (sedimentary) — marble is the metamorphosed version
• Thinking faster cooling always produces larger crystals — it is the opposite: slow cooling = large crystals
• Saying weathering and erosion are the same — weathering breaks rock down in place; erosion transports the fragments
• Thinking metamorphic rock has melted — it transforms by heat/pressure without melting (melting produces igneous rock)

Tips and Tricks

• Remember the three rock types: I-S-M (Igneous = heat/cooling; Sedimentary = layers/compaction; Metamorphic = heat+pressure)
• Crystal size memory trick: slow = big (more time to grow); fast = tiny
• Fossil rule: fossils are only found in sedimentary rocks because metamorphic and igneous processes destroy soft tissues
• Mohs scale: fingernail (~2.5), copper coin (~3.5), glass (~5.5), steel file (~6.5) — useful for field testing