---
title: "Minerals and the Three Rock Families"
description: "A rock is an assemblage of minerals, and which minerals are present plus how they are arranged says how the rock formed. Grain size in an igneous rock records cooling rate, sorting in a sedimentary on"
canonical: https://lightmysky.com/learn/science/minerals-and-the-three-rock-families-mt_S0edEKh084
source: https://lightmysky.com/learn/science/minerals-and-the-three-rock-families-mt_S0edEKh084.md
retrieved: 2026-09-12
---

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# Minerals and the Three Rock Families

A rock is an assemblage of minerals, and which minerals are present plus how they are arranged says how the rock formed. Grain size in an igneous rock records cooling rate, sorting in a sedimentary one records transport, and banding in a metamorphic one records directed pressure.

Subject: Science · Area: Earth's Systems · Ages 14 to 15
Page: https://lightmysky.com/learn/science/minerals-and-the-three-rock-families-mt_S0edEKh084

## Ready when they can

- Identify a hand specimen as igneous, sedimentary or metamorphic from texture rather than colour
- Explain what crystal size in an igneous rock tells you about where it cooled
- Relate the sorting and rounding of sediment grains to how far they travelled

## Lesson: Reading a rock like a geologist

A rock is a pack of minerals grown or glued together. Which minerals sit inside, and how they are arranged, tells you how the rock formed. That is why you judge a rock by its texture and never by its colour.

**Example.** Picture four mystery rocks. Granite holds interlocking crystals that grew as melt cooled. Sandstone and conglomerate hold separate grains and pebbles pressed and glued together. Phyllite shines because its minerals were pressed flat into lined up sheets.

Each family hides a measuring tape. In an igneous rock, big crystals mean the melt cooled slowly deep down, and tiny ones mean it cooled fast at the surface. In a sedimentary rock, rounded well sorted grains travelled far, while angular mixed grains stayed near home. In a metamorphic rock, bands record pressure that pushed harder from one side.

**Tip.** Hold any hand specimen to the light and run this check. Interlocking crystals mean igneous. Loose looking grains or pebbles mean sedimentary. Shine plus bands plus splitting into flat sheets mean metamorphic.

**Recap.** Minerals plus their arrangement name the family, and grain size, sorting and banding tell where and how each rock formed.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Plate Tectonics: the Evidence and the Engine](https://lightmysky.com/learn/science/plate-tectonics-the-evidence-and-the-engine-mt_K4hJlyOZ7p)
- [The Rock Cycle](https://lightmysky.com/learn/science/the-rock-cycle-mt_NckKLZ3uCE)

## Opens up

- [Reading a Rock Sequence: Superposition, Fossils and Correlation](https://lightmysky.com/learn/science/reading-a-rock-sequence-superposition-fossils-and-correlation-mt_gIwqGoQsag)