---
title: "Rocks and Minerals"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 15
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 15 — Rocks and Minerals

Sam leans over the kitchen worktop with a hand lens. The polished stone, which looks grey from far away, turns into a crowd of specks: pink ones, white ones, glassy grey ones and tiny black ones that shine when the light catches them. Each speck has flat, shiny faces, and the specks fit together like the pieces of a jigsaw. What are they?

![Looking at a speckled stone worktop through a hand lens. (AI illustration.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-7e8e45b4b43e.jpg)

*Looking at a speckled stone worktop through a hand lens. (AI illustration.)*

**You already know.**

[Rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are the hard material of the Earth’s crust: they can be broken into stones, rolled by [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) into [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), and ground into [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) ([Chapter 5](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#ch-g3-stones-pebbles-sand)). Water, ice and wind wear [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) away ([weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering)), carry the pieces off ([erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion)) and drop them further down ([deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition)) ([Chapter 10](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#ch-g5-water-shapes-the-land)). At a [volcano](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-volcano), molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) comes out as [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption) ([Chapter 8](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#ch-g4-volcanoes-earthquakes-first-look)).

## 15.1 A rock is made of minerals

The specks of the worktop are [minerals](#def-g6-rocks-and-minerals-mineral), and the stone is a granite.

**Definition 15.1 (Mineral).**

A *mineral* is a natural solid substance, not made by living things, always with the same make-up and with its atoms arranged in a regular way. A *[rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)* is made of one mineral or, more often, of several minerals stuck together.

Granite is made mostly of three [minerals](#def-g6-rocks-and-minerals-mineral): *feldspar* (pink or white), *quartz* (grey and glassy) and *mica* (black or silvery, in thin shiny flakes). Other [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are made of a single [mineral](#def-g6-rocks-and-minerals-mineral): limestone, for example, is made almost only of calcite.

![Left: granite drawn as a mosaic of grains, each grain one mineral (an enlargement, not to scale). Right: a real pink granite, about the size of a hand. (Photo: James St. John, CC BY 2.0, via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-8e912dca26f4.svg)

![](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-211a90f02424.svg)

![](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-f080a0cb0272.svg)

![](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-7026984c7027.svg)

![Left: granite drawn as a mosaic of grains, each grain one mineral (an enlargement, not to scale). Right: a real pink granite, about the size of a hand. (Photo: James St. John, CC BY 2.0, via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-0d9ff2d546ae.jpg)

*![](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-211a90f02424.svg) feldspar ![](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-f080a0cb0272.svg) quartz ![](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-7026984c7027.svg) mica*

*Left: granite drawn as a mosaic of grains, each grain one [mineral](#def-g6-rocks-and-minerals-mineral) (an enlargement, not to scale). Right: a real pink granite, about the size of a hand. (Photo: James St. John, CC BY 2.0, via Wikimedia Commons.)*

## 15.2 Crystals

Under the lens, many [mineral](#def-g6-rocks-and-minerals-mineral) grains show flat, shiny faces and straight edges. Where a [mineral](#def-g6-rocks-and-minerals-mineral) has had room to grow freely, in a hollow of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), it forms a crystal: a solid with a regular shape bounded by flat faces. Its shape comes from the regular arrangement of its atoms, studied in the chemistry books. A quartz crystal, for example, is a column with six sides ending in a point; a mica crystal splits into sheets thinner than paper.

![Left: a quartz crystal, with its flat faces. Right: a sheet of mica, a mineral that splits into very thin flakes. (Photos: James St. John, CC BY 2.0, via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-8a982f9f46c1.jpg)

![Left: a quartz crystal, with its flat faces. Right: a sheet of mica, a mineral that splits into very thin flakes. (Photos: James St. John, CC BY 2.0, via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-824b2da6926e.jpg)

*Left: a quartz crystal, with its flat faces. Right: a sheet of mica, a [mineral](#def-g6-rocks-and-minerals-mineral) that splits into very thin flakes. (Photos: James St. John, CC BY 2.0, via Wikimedia Commons.)*

In granite, the grains grew pressed against each other, so they are not nicely shaped: they fit together like a jigsaw. Their size tells a story. Granite forms deep underground, where molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) cools very slowly: its grains have time to grow large enough to be seen with the eye. Molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that cools quickly at the surface, as [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption), gives [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) with grains too small to see.

## 15.3 The three rock families

Geologists sort [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) into three families, not by their colour, but by the way they formed.

**Definition 15.2 (The three rock families).**

An *igneous rock* forms when molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) cools and becomes solid: granite deep underground, basalt from [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption). A *sedimentary rock* forms at the Earth’s surface from grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), or from the shells of living things, laid down in layers and then hardened: sandstone, shale, limestone. A *metamorphic rock* is a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that has been transformed, without melting, by heat and pressure deep underground: marble (from limestone), slate (from shale).

The name *igneous* comes from the Latin word for fire. [Sedimentary rocks](#def-g6-rocks-and-minerals-rock-families) often show layers, one on top of the other, like the pages of a book. [Metamorphic rocks](#def-g6-rocks-and-minerals-rock-families) are often squeezed and folded; some, like slate, split into flat sheets.

Many [sedimentary rocks](#def-g6-rocks-and-minerals-rock-families) are made of the grains you sorted by size in [Chapter 5](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#ch-g3-stones-pebbles-sand): [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). Once buried, the grains lose their water and are stuck together by a natural cement. The size of the grains gives the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) its name.

![Three sedimentary rocks named after the size of their grains (grains much enlarged; the sand shown at the scale of the box, the gravel larger).](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-d39810228316.svg)

*Three [sedimentary rocks](#def-g6-rocks-and-minerals-rock-families) named after the size of their grains (grains much enlarged; the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) shown at the scale of the box, the [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) larger).*

**In the field — Reading the stones of a town.**

Towns are open-air [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) collections. Walk with an adult past old walls, steps, statues, church fronts and paving, and look closely, without scratching or breaking anything. Some kerbs and steps are granite: look for its glassy quartz and shiny mica. Many old walls and fronts are limestone or sandstone: look for shells, or rub a fingertip on a fallen crumb of sandstone to feel the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). Roofs may be slate, split into thin sheets. Note in a notebook where each stone is, its colour, its grains, and the family you think it belongs to; a photograph helps to compare later with the table of this chapter.

## 15.4 The rock cycle

[Rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are not eternal. A granite [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) is slowly weathered; its grains are carried by [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) and laid down there in layers; buried deeper and deeper, the layers harden into sandstone. If the sandstone is carried still deeper, heat and pressure transform it into a [metamorphic rock](#def-g6-rocks-and-minerals-rock-families); deeper still, it can melt, and the molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), cooling, makes a new [igneous rock](#def-g6-rocks-and-minerals-rock-families). Little by little, over millions of years, the material of the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) goes round and round.

**Definition 15.3 (Rock cycle).**

The *rock cycle* is the endless journey by which [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of each family are slowly changed into [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the other families: by [weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering) and [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) at the surface, by burial and hardening, by heat and pressure, by melting and cooling.

![The rock cycle. Each arrow takes from thousands to millions of years. Any rock that comes up to the surface can be weathered (grey arrows).](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-f7b2c475e0da.svg)

*The [rock cycle](#def-g6-rocks-and-minerals-rock-cycle). Each arrow takes from thousands to millions of years. Any [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that comes up to the surface can be weathered (grey arrows).*

## 15.5 Recognising common rocks

Here are seven [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that are often met in buildings, pavements and [landscapes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape).

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-0d9ff2d546ae.jpg)

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-b6a16c0b81bc.jpg)

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-547a6487fc5b.jpg)

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-c04b163943a2.jpg)

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-10129bb99fcf.jpg)

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-6acffd012fd3.jpg)

![Seven common rocks. (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/img-9488482f6441.jpg)

*granite*

*basalt*

*sandstone*

*limestone (with shells)*

*shale*

*marble*

*slate, on a roof*

*Seven common [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). (Photos: James St. John, CC BY 2.0; roof: Øyvind Holmstad, CC0; via Wikimedia Commons.)*

| rock | family | what to look for |
| --- | --- | --- |
| granite | igneous | large grains of several colours: pink or white feldspar, grey glassy quartz, black shiny mica |
| basalt | igneous | dark grey to black, grains too small to see, often small round holes left by gas bubbles |
| sandstone | sedimentary | grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) you can feel, rough like sandpaper |
| limestone | sedimentary | pale, often with shells; fizzes under a drop of dilute acid |
| shale | sedimentary | very fine and soft, made of hardened [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) (clay); in thin layers |
| marble | metamorphic | white or coloured, made of shiny crystals that sparkle like sugar; fizzes under acid |
| slate | metamorphic | grey, very fine, splits into thin, hard, flat sheets |

**Safety.**

The acid test is done only by an adult, with goggles and gloves, using a very dilute acid. Never taste a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) or a powder, and never break [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) with a tool: looking through a hand lens tells a great deal.

A key is a list of questions, each with two answers, that leads step by step to the name of a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

![A key to the seven rocks of the table. Start at the top and answer each question with yes or no.](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-794db888349f.svg)

*A key to the seven [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the table. Start at the top and answer each question with yes or no.*

**Hardness.** One more clue helps to recognise a [mineral](#def-g6-rocks-and-minerals-mineral): how easily it can be scratched.

**Definition 15.4 (Hardness and the Mohs scale).**

The *hardness* of a [mineral](#def-g6-rocks-and-minerals-mineral) is how well it resists being scratched: a [mineral](#def-g6-rocks-and-minerals-mineral) scratches every softer one and is scratched by every harder one. The *Mohs scale* ranks hardness from 1 to 10 with ten reference [minerals](#def-g6-rocks-and-minerals-mineral), from talc (1), the softest, to diamond (10), the hardest.

![The Mohs scale (left: the ten reference minerals) and four everyday testers (right, in red).](https://one-course.com/images/onecourse/chapters/earth-1/g6-rocks-and-minerals/fig-f96a2e297d6e.svg)

*The [Mohs scale](#def-g6-rocks-and-minerals-hardness) (left: the ten reference [minerals](#def-g6-rocks-and-minerals-mineral)) and four everyday testers (right, in red).*

**Method 15.5 (The scratch test).**

1. Try to scratch the [mineral](#def-g6-rocks-and-minerals-mineral) with your fingernail, then with a copper coin. (Glass and steel are used only by an adult.)
2. Rub the mark with a finger: a true scratch stays, a trace of powder from the tester rubs off.
3. The [hardness](#def-g6-rocks-and-minerals-hardness) is between that of the hardest tester that does not scratch it and that of the softest tester that does.

**Example 15.6 (Quartz or calcite?).**

Two white [minerals](#def-g6-rocks-and-minerals-mineral) look alike. The first one is scratched by a copper coin: its [hardness](#def-g6-rocks-and-minerals-hardness) is less than 3.5; it is calcite (3). The second one is not scratched by a steel nail and scratches glass: its [hardness](#def-g6-rocks-and-minerals-hardness) is more than 6.5; among the reference [minerals](#def-g6-rocks-and-minerals-mineral), it is quartz (7).

## 15.6 Exercises

**Exercise 15.1 ★.**

To which family does each [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) belong: granite, sandstone, marble, basalt, limestone, slate?

**Solution of Exercise 15.1.**

Granite: igneous. Sandstone: sedimentary. Marble: metamorphic. Basalt: igneous. Limestone: sedimentary. Slate: metamorphic.

**Exercise 15.2 ★.**

Is quartz a [mineral](#def-g6-rocks-and-minerals-mineral) or a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? And granite? Explain the difference in one sentence.

**Solution of Exercise 15.2.**

Quartz is a [mineral](#def-g6-rocks-and-minerals-mineral); granite is a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). A [mineral](#def-g6-rocks-and-minerals-mineral) is a single natural substance with its atoms arranged in a regular way; a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is made of one or several [minerals](#def-g6-rocks-and-minerals-mineral) stuck together (granite: feldspar, quartz and mica).

**Exercise 15.3 ★.**

Put these [minerals](#def-g6-rocks-and-minerals-mineral) in order, from the softest to the hardest: quartz, talc, diamond, calcite. Which of them can scratch all the others?

**Solution of Exercise 15.3.**

Talc (1), calcite (3), quartz (7), diamond (10). Diamond, the hardest, scratches all the others.

**Exercise 15.4 ★.**

Copy and complete with the words *cooling*, *heat and pressure*, *[weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering)*: “Molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) becomes an [igneous rock](#def-g6-rocks-and-minerals-rock-families) by … A limestone becomes a marble by … A granite turns into grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) by …”

**Solution of Exercise 15.4.**

“Molten [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) becomes an [igneous rock](#def-g6-rocks-and-minerals-rock-families) by cooling. A limestone becomes a marble by heat and pressure. A granite turns into grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) by [weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering).”

**Exercise 15.5 ★★.**

Use the key. (a) A [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) does not fizz under acid; it is dark, with grains too small to see and small round holes. (b) A [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) fizzes under acid and is made of shiny crystals that sparkle like sugar. (c) A [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) does not fizz; it is rough like sandpaper. Name each [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**Solution of Exercise 15.5.**

(a) Basalt. (b) Marble. (c) Sandstone.

**Exercise 15.6 ★★.**

A [mineral](#def-g6-rocks-and-minerals-mineral) is scratched by a copper coin but not by a fingernail. Between which two [hardness](#def-g6-rocks-and-minerals-hardness) values is it? Which reference [mineral](#def-g6-rocks-and-minerals-mineral) of the [Mohs scale](#def-g6-rocks-and-minerals-hardness) could it be?

**Solution of Exercise 15.6.**

Its [hardness](#def-g6-rocks-and-minerals-hardness) is more than that of a fingernail (about 2.5) and less than that of a copper coin (about 3.5). It could be calcite (3).

**Exercise 15.7 ★★.**

In the drawing of granite in this chapter, there are 60 grains: 33 of feldspar, 21 of quartz and 6 of mica. What percentage of the grains are quartz? Feldspar? Mica? Check that the three percentages add up to 100 %.

**Solution of Exercise 15.7.**

Quartz: $21 \div 60 = 0.35$, that is 35 %. Feldspar: $33 \div 60 = 0.55$, that is 55 %. Mica: $6 \div 60 = 0.10$, that is 10 %. Check: $35 + 55 + 10 = 100$.

**Exercise 15.8 ★★.**

Granite and basalt are both [igneous rocks](#def-g6-rocks-and-minerals-rock-families). Why can we see the grains of granite with our eyes, but not those of basalt?

**Solution of Exercise 15.8.**

Granite cools very slowly, deep underground: its grains have a long time to grow, and become large. Basalt comes from [lava](https://one-course.com/books/earth/1/en/chapter/8-volcanoes-and-earthquakes-a-first-look#def-g4-volcanoes-earthquakes-first-look-eruption), which cools quickly at the surface: its grains have no time to grow and stay too small to see.

**Exercise 15.9 ★★.**

Marble is made from limestone. (a) To which family does each belong? (b) What happened to the limestone to turn it into marble? (c) Why do both fizz under acid?

**Solution of Exercise 15.9.**

(a) Limestone is sedimentary; marble is metamorphic. (b) Deep underground, heat and pressure transformed it without melting it: its calcite grew into shiny crystals that fit together. (c) Both are made of the same [mineral](#def-g6-rocks-and-minerals-mineral), calcite, and it is calcite that fizzes under acid.

**Exercise 15.10 ★★.**

Using the [Mohs scale](#def-g6-rocks-and-minerals-hardness): (a) can quartz scratch a glass bottle? (b) Can fluorite? (c) Can a steel nail scratch feldspar?

**Solution of Exercise 15.10.**

(a) Yes: quartz (7) is harder than glass (about 5.5). (b) No: fluorite (4) is softer than glass. (c) Yes: a steel nail (about 6.5) is harder than feldspar (6).

**Exercise 15.11 ★★★.**

Tell the journey of a grain of quartz, in five steps, from a granite [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) to a [metamorphic rock](#def-g6-rocks-and-minerals-rock-families). Name the process at each step, and the family of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) at the start and at the end.

**Solution of Exercise 15.11.**

For example: (1) the granite of the [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) (igneous) is broken up by [weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering), and its quartz grains are set free; (2) a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) carries them off: [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion); (3) the grains are laid down in layers at the bottom of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition); (4) buried under other layers, they are pressed and cemented into a sandstone: hardening (the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is now sedimentary); (5) carried deep underground, the sandstone is transformed by heat and pressure into a [metamorphic rock](#def-g6-rocks-and-minerals-rock-families).

**Exercise 15.12 ★★★.**

A grey glassy [mineral](#def-g6-rocks-and-minerals-mineral) is not scratched by a steel nail, scratches glass, and is scratched by topaz. (a) What is its [hardness](#def-g6-rocks-and-minerals-hardness) on the [Mohs scale](#def-g6-rocks-and-minerals-hardness)? Name it. (b) Grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) are rolled for years by waves and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). Using its [hardness](#def-g6-rocks-and-minerals-hardness), explain why grains of this [mineral](#def-g6-rocks-and-minerals-mineral) last much longer than grains of calcite or mica.

**Solution of Exercise 15.12.**

(a) Not scratched by steel (about 6.5) and able to scratch glass: harder than 6.5. Scratched by topaz (8): softer than 8. Its [hardness](#def-g6-rocks-and-minerals-hardness) is 7: it is quartz. (b) Quartz is harder than almost everything that rubs against it (other grains, [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble)): it is hardly worn away. Calcite (3) and mica (2 to 3) are much softer: rubbing grinds them into tiny pieces that are carried away.

## 15.7 Problem: The stones of the town square

**Problem 15.1.**

Weekend problem — the rocks of a town square, the hardest mineral among them, and how many rock families are present

On a school trip, the class observes the stones of the old town square (observing only: nobody breaks or scratches the monuments). The teacher has brought loose pieces of the same stones from the stonemason’s yard for the [hardness](#def-g6-rocks-and-minerals-hardness) tests.

**The fountain**: its basin shows large grains, pink, white, grey and glassy, and black and shiny. **The paving**: pale slabs in which you can see small fossil shells; a drop of dilute acid, put by the teacher on a loose piece, fizzes. **The roof of the town hall**: thin, flat, grey sheets of a very fine-grained stone. **The statue**: a white stone made of shiny crystals that sparkle like sugar; it fizzes too.

**Part I — The fountain, the paving, the roof.**

1. Name the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the fountain and its family. Name its three [minerals](#def-g6-rocks-and-minerals-mineral) .
2. Name the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the paving and its family. What were the shells before they were in the stone?
3. Name the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the roof and its family. From which [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) did it form?
4. Name the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of the statue and its family. From which [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) did it form?

**Part II — A hardness test.**

5. The teacher tries to scratch a grey glassy grain of the fountain with a steel nail: no scratch. The grain scratches glass. What is this [mineral](#def-g6-rocks-and-minerals-mineral) , and what is its [hardness](#def-g6-rocks-and-minerals-hardness) ?
6. The pink grains are scratched by the steel nail but not by glass. Between which two values is their [hardness](#def-g6-rocks-and-minerals-hardness) ? Which [mineral](#def-g6-rocks-and-minerals-mineral) of the [Mohs scale](#def-g6-rocks-and-minerals-hardness) is it?
7. The black flakes can just be scratched by a fingernail. Which [mineral](#def-g6-rocks-and-minerals-mineral) is it?
8. Can a copper coin scratch the paving? Explain with the [hardness](#def-g6-rocks-and-minerals-hardness) of calcite.

**Part III — The rock cycle.**

9. After centuries of [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) , the statue has lost its fine details. Which process of the [rock cycle](#def-g6-rocks-and-minerals-rock-cycle) is at work?
10. The granite of the fountain came from a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) . Over millions of years, what could become of the quartz grains of that [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) , and in which [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) could they end up?
11. How many of the three [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) families are present in the square? Name the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) or [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of each.
12. What is the Mohs [hardness](#def-g6-rocks-and-minerals-hardness) of the hardest [mineral](#def-g6-rocks-and-minerals-mineral) found in the stones of the square?

**Solution of Problem 15.1.**

**1.** Granite, an [igneous rock](#def-g6-rocks-and-minerals-rock-families); its [minerals](#def-g6-rocks-and-minerals-mineral) are feldspar (pink or white), quartz (grey, glassy) and mica (black, shiny).

**2.** Limestone, a [sedimentary rock](#def-g6-rocks-and-minerals-rock-families). The shells belonged to animals that lived in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); their remains were laid down and hardened into the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**3.** Slate, a [metamorphic rock](#def-g6-rocks-and-minerals-rock-families), formed from shale.

**4.** Marble, a [metamorphic rock](#def-g6-rocks-and-minerals-rock-families), formed from limestone.

**5.** Quartz: not scratched by steel (about 6.5), it scratches glass (about 5.5); its [hardness](#def-g6-rocks-and-minerals-hardness) is 7.

**6.** Scratched by steel (about 6.5) but not by glass (about 5.5): between 5.5 and 6.5. It is feldspar (orthoclase, 6).

**7.** Mica ([hardness](#def-g6-rocks-and-minerals-hardness) 2 to 3).

**8.** Yes: the paving is limestone, made of calcite (3), softer than a copper coin (about 3.5).

**9.** [Weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering): the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) slowly wears the surface of the stone away.

**10.** The [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) is weathered; its quartz grains are carried by [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and laid down in layers, at the bottom of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); buried and hardened, they can become a sandstone, a [sedimentary rock](#def-g6-rocks-and-minerals-rock-families).

**11.** All three families are present: igneous (the granite of the fountain), sedimentary (the limestone of the paving), metamorphic (the slate of the roof and the marble of the statue).

**12.** The hardest [mineral](#def-g6-rocks-and-minerals-mineral) is the quartz of the granite: Mohs [hardness](#def-g6-rocks-and-minerals-hardness) **7**.