Investigate how minerals form by crystallisation and how their physical and chemical properties, including crystal structure, hardness, cleavage and composition, are used to identify them in the Australian context

What this dot point is asking

NESA wants you to define a mineral, explain how minerals crystallise from melts and solutions, and use diagnostic physical and chemical properties to identify common minerals. You should be able to apply identification properties (hardness, cleavage, lustre, streak, crystal form) and anchor the topic with an Australian mineral.

The answer

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and an ordered internal atomic structure (a crystal lattice). Rocks are made of minerals, so understanding minerals underpins the whole module. The orderly atomic arrangement is what gives each mineral its characteristic, reproducible properties.

How minerals form

Minerals form mainly by crystallisation, the growth of an ordered solid from a disordered state. There are three common settings. Crystallisation from a melt occurs as magma or lava cools, with different minerals freezing out at different temperatures (the basis of the magmatic ore processes in this module). Crystallisation from solution occurs when water evaporates or cools and can no longer hold dissolved ions, precipitating minerals such as halite, gypsum and the silica that forms Australian opal at Coober Pedy and Lightning Ridge. Recrystallisation in the solid state occurs during metamorphism, growing new minerals such as garnet and mica. Slow growth with space produces large, well-formed crystals; rapid growth or crowding produces small, irregular grains.

The silicate framework

Most rock-forming minerals are silicates, built from the silica tetrahedron, a unit of one silicon atom bonded to four oxygen atoms. The way these tetrahedra link, as isolated units, chains, sheets or three-dimensional frameworks, defines the major silicate groups (olivine, pyroxene, amphibole, mica, feldspar and quartz). Silicon and oxygen are the two most abundant elements in the crust, so silicates dominate Australian rocks.

Identifying minerals by their properties

Because structure controls properties, a few simple tests identify most common minerals.

• Hardness, the resistance to scratching, is ranked on Mohs' scale from talc (1) to diamond (10); quartz (7) scratches glass.
• Cleavage is the tendency to break along flat planes of weak bonding; mica peels into sheets, while quartz has no cleavage and instead fractures.
• Lustre describes how the surface reflects light (metallic, glassy, dull).
• Streak is the colour of the powdered mineral, rubbed on a tile; haematite gives a red-brown streak even when the lump looks grey.
• Crystal form is the external shape produced by the internal lattice, such as the six-sided prisms of quartz.
• Other diagnostic tests include colour (often unreliable on its own), density, magnetism (magnetite) and reaction with acid (calcite fizzes).

Australian context

Australia produces and hosts many notable minerals: opal (the national gemstone), sapphire from the New England and central Queensland fields, diamond from the former Argyle mine in the Kimberley, and the iron, aluminium and titanium minerals mined elsewhere in this module. Distinguishing a valuable mineral from a worthless look-alike, for example gold from pyrite (fool's gold, which is harder, brittle and gives a dark streak), is a practical use of these identification tests.

Try this

Q1. Define a mineral and state two reasons opal does not strictly meet the definition. [3 marks]

• Cue. A mineral is a naturally occurring inorganic solid with definite composition and ordered structure; opal lacks an ordered crystal lattice, so it is a mineraloid.

Q2. A student has two grey samples and must decide which is haematite. Describe two tests that would distinguish it. [3 marks]

• Cue. Streak (haematite gives a red-brown streak) and density or magnetism would separate haematite from a colourless look-alike.