Rocks and Minerals

Lessons

Senior 6 · Geography

Rocks and Minerals

4 lessons

Lesson 1 of 4

Minerals: Properties and Identification

25 min

Why the black grains stay in the pan

A miner swirling crushed rock and water in a shallow pan washes the light grains over the edge, and a few heavy, dark grains stay behind. In Rwanda those heavy grains are often cassiterite, the ore of tin, which is about two and a half times as dense as the quartz around it. The miner is using a physical property of a mineral, its density, to separate it. Every mineral has a set of such properties, and a geologist with a pocket knife, a streak plate, a hand lens and a bottle of dilute acid can identify most common minerals in the field. This lesson explains what minerals are and how they are told apart.

Definition

Mineral: a naturally occurring, inorganic solid with a definite chemical composition and an ordered internal arrangement of atoms (a crystal structure). Example: quartz, $\mathrm{SiO_2}$ .

Rock: a naturally formed solid made of one or more minerals (or of glass or organic matter). Example: granite is a rock made mainly of quartz, feldspar and mica.

Rock-forming minerals: the few minerals that make up most rocks, chiefly feldspars, quartz, micas, amphiboles, pyroxenes, olivine and calcite.

Types of minerals

Minerals are grouped by the chemical part they share (their anion group).

Group	Shared part	Examples
Silicates (most rocks)	$\mathrm{SiO_4}$ units	Quartz, feldspar, mica, olivine
Carbonates	$\mathrm{CO_3}$	Calcite $\mathrm{CaCO_3}$ (limestone, marble)
Oxides	O	Cassiterite $\mathrm{SnO_2}$ , haematite $\mathrm{Fe_2O_3}$ , columbite-tantalite (coltan)
Sulfides	S	Pyrite $\mathrm{FeS_2}$ , galena $\mathrm{PbS}$
Tungstates	$\mathrm{WO_4}$	Wolframite, the main ore of tungsten
Native elements	a single element	Gold, copper, diamond (carbon)

Physical properties

Property	What it means	Example
Colour	The colour seen in light; often unreliable because small impurities change it	Quartz can be clear, white, pink or purple
Streak	The colour of the powdered mineral, seen by rubbing it on unglazed porcelain; far more reliable than colour	Haematite is black or red but always has a red-brown streak
Lustre	How the surface reflects light: metallic, glassy (vitreous), pearly, silky, dull	Galena is metallic; quartz is glassy
Hardness	Resistance to scratching, on the Mohs scale of 1 to 10 (Figure 1)	Quartz (7) scratches glass; calcite (3) does not
Cleavage and fracture	Cleavage: splitting along flat planes of weakness. Fracture: irregular or curved (conchoidal) breaking	Mica splits into thin sheets; quartz breaks with a conchoidal fracture
Specific gravity (SG)	Density compared with water	Quartz 2.65; cassiterite about 7
Crystal form (habit)	The shape in which crystals grow	Quartz forms six-sided prisms; pyrite forms cubes
Magnetism	Attraction to a magnet	Magnetite is strongly magnetic

Figure 1: the Mohs scale of hardness.

Worked example 1: is it cassiterite?

Problem. A dark, heavy grain weighs 52.0 g in air and 44.6 g when hung in water. Find its specific gravity and decide whether it is more likely cassiterite (SG about 6.8–7.1) or a dark quartz grain (SG 2.65).

Solution. The loss of weight in water equals the weight of water it displaces, so

\mathrm{SG} = \frac{\text{weight in air}}{\text{weight in air} - \text{weight in water}} = \frac{52.0}{52.0 - 44.6} = \frac{52.0}{7.4} = 7.0

An SG of 7.0 fits cassiterite and rules out quartz. This high density is exactly why cassiterite stays in the miner's pan.

Worked example 2: identifying by elimination

Problem. A pale, glassy mineral cannot be scratched by a fingernail, but a copper coin and a steel knife both scratch it. It fizzes strongly in cold dilute hydrochloric acid and breaks into rhomb-shaped pieces bounded by three sets of flat faces. Name it.

Solution.

Hardness: harder than a fingernail (2.5) but softer than a coin (3.5), so about 3.
Strong fizzing in cold acid shows a carbonate, most likely calcite.
Three cleavages that are not at right angles give rhomb-shaped fragments, which is typical of calcite.

All three tests agree: the mineral is calcite ( $\mathrm{CaCO_3}$ ). Quartz is ruled out because it would scratch the knife and does not react with acid.

Definition

Chemical properties of a mineral describe how it reacts: its composition, its reaction with acid, and how easily it dissolves or oxidises.

Chemical properties

Reaction with acid. Carbonates fizz in dilute hydrochloric acid as carbon dioxide is released: $\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2}$ Calcite fizzes strongly; dolomite only when powdered.
Solubility. Halite (rock salt, $\mathrm{NaCl}$ ) dissolves in water and tastes salty; gypsum dissolves slowly.
Oxidation. Iron-bearing minerals such as pyrite and olivine react with oxygen and water, forming rust-coloured iron oxides; this is why weathered rocks and many tropical soils are red.
Stability. Quartz is chemically very resistant, so it survives weathering to become sand; feldspar breaks down by hydrolysis to clay.

Warning

Common mistakes

Identifying by colour alone. Use streak, hardness and cleavage; colour misleads.
Reading a scratch test backwards. If A scratches B, A is the harder. Wipe the mark to check it is a real groove, not powder left by a softer mineral.
Safety: use only dilute acid, wear eye protection, and wash spills off skin with plenty of water.

Tip

Memory hook for Mohs: "Tall Giraffes Can Find Apples On Quite Tall Cherry Dates" for Talc, Gypsum, Calcite, Fluorite, Apatite, Orthoclase, Quartz, Topaz, Corundum, Diamond.

Geoscience Australia: how the streak test is done and why streak is more reliable than colour.

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Practice

Define a mineral and distinguish it from a rock.
Name the mineral group of each: calcite, cassiterite, pyrite, quartz.
Explain why streak is more useful than colour for identifying minerals.
A mineral scratches glass but is scratched by topaz. Between which values does its Mohs hardness lie?
A sample weighs 39.6 g in air and 24.6 g in water. Calculate its specific gravity and suggest whether it could be quartz.
Describe a test to show that a rock contains calcite, and write the equation.
Explain why quartz is common in river sand but feldspar is rare.

Show answersHide answers

A mineral is a natural inorganic solid with a definite composition and crystal structure; a rock is a natural solid made of one or more minerals.
Carbonate; oxide; sulfide; silicate.
Colour varies with small impurities, but the colour of the powder (streak) stays the same for a mineral.
Above about 5.5 (glass) and below 8 (topaz).
$\mathrm{SG} = \frac{39.6}{39.6 - 24.6} = \frac{39.6}{15.0} = 2.64$ ; yes, this matches quartz (2.65).
Add a drop of cold dilute hydrochloric acid: calcite fizzes. $\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2}$ .
Quartz is hard and chemically resistant, so it survives weathering and transport; feldspar breaks down by hydrolysis to clay.

Summary

A mineral has a definite composition and crystal structure; rocks are made of minerals.
Minerals are grouped as silicates, carbonates, oxides, sulfides, tungstates and native elements.
Streak, hardness, cleavage, lustre and specific gravity identify minerals better than colour.
Chemical tests include fizzing in acid, solubility and oxidation.

Sources and credits

Mineral, Mohs scale and Cassiterite: Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa.

Photos: Cassiterite - Mt Bischoff mine (Ralph Bottrill, CC BY 3.0); Wolframite with quartz (Shannon Heinle, CC0); both via Wikimedia Commons.

Figure 1, the worked examples and the exercises were drawn and written by Ganzaa.

Video: "Identifying Minerals - Streak Test", Geoscience Australia, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why the black grains stay in the pan

2Types of minerals

3Physical properties

4Chemical properties

Why the black grains stay in the pan

Types of minerals

Physical properties

Chemical properties