The term mineral is used to describe a naturally occurring solid substance with a reasonably fixed chemical composition and crystal structure.
Minerals can be identified based on a number of properties. The properties most commonly used in identification of a mineral are colour, streak, lustre, hardness, crystal shape, cleavage, specific gravity and habit. Most of these can be assessed relatively easily even when a geologist is out in the field.
Many mineral names end in ‘ite’. This suffix is derived from the Greek word lithos (from its adjectival form -ites), meaning rock or stone.
Every mineral has two different names: >Mineralogical, e.g. pyrite, quartz.
>Chemical, e.g. iron sulphide FeS2 or silicon dioxide SiO2.
Well-formed crystals of some minerals will also have a gemmological name, e.g. peridot is clear, gemstone quality olivine.
The mineral corundum (aluminium oxide – Al2O3) is colourless when pure but when it contains chromium and iron it is a red colour and is called a ruby.
When corundum contains iron and titanium it is blue and is called a sapphire.
Pyrite (fool's gold) has a streak colour very different from the colour of the mineral. Pyrite will leave a black powder if it is scratched on a white tile, whereas real gold will leave a yellow/gold smear.
The streak of malachite is green.
Galena has a steel grey streak.
Pure sphalerite has a white streak. However, impurities are almost always present, giving this mineral a light brown streak. The streak will always be a lighter colour than the specimen.
The streak of hematite is red to reddish brown.
Galena has a metallic lustre.
Sphalerite has a resinous lustre.
Quartz has a vitreous lustre. This means it looks like broken glass.
Muscovite mica has a pearly lustre.
Talc has an earthy lustre which is dull with no shine.
Hardness can sometimes vary in a sample depending on the direction in which the mineral is scratched (which bonds are broken within the crystal structure). For example, when kyanite is scratched in one direction it exhibits a hardness of 4 to 5 but when scratched in a perpendicular direction it exhibits a hardness of 6 to 7.
Hexagonal outline of ruby crystal.
Dodecahedral almandine (type of garnet) crystal.
Cubic pyrite crystal. Iron pyrite (FeS2) is often found as cubic crystals known by old time miners in Western Australia as “Devil’s dice”.
Octahedral fluorite crystal.
Calcite (CaCO3) has regular cleavage at 60° and 120° resulting in beautiful rhombic crystals.
Halite (rock salt) has cleavage in three directions, resulting in cubic crystals.
Quartz does not have any planes of weakness so does not cleave (split along planes). When it breaks it is said to fracture, leaving glassy conchoidal (curved) surfaces.
Stone Age people knew that quartz and some silica-rich rocks produced very sharp edges when fractured. They learnt to make stone tools such as knives and arrow heads from rocks such as chert, obsidian and flint.
Fishtail twinned calcite.
Hefting is how heavy a mineral feels in the hand, an informal evaluation of density. Most minerals are about three times as dense as water, that is, they have a specific gravity of about 3. Galena is distinctly heavy, with a specific gravity of 7.4 to 7.6.
This specimen of natrolite displays an acicular habit: needle-like, slender and/or tapered crystals.
Rhodonite with a columnar habit: long, slender prisms often with parallel growth.
Biotite mica displaying a foliated or lamellar habit: layered structure, parting into thin sheets.
This specimen of mimetite has a globular or botryoidal habit: grape-like, hemispherical masses.
These crystals of tourmaline have a prismatic habit: elongate, prism-like crystals.
Desert rose has a rosette or lenticular habit: platy, radiating rose-like aggregate.
This inesite has a stellate habit: star-like, radiating crystals.
This sample of amethyst is translucent.
Malachite is opaque.
Classroom experiment testing electrical conductivity of pyrite - the bulb illuminates.
The National Mineral and Commonwealth Paleontological Collection, Geoscience Australia and The National Museum of Australia Mineral Collections (specimens)
Chris Fitzgerald (photography)
Katy Buffinton, Shona Blewett and Ngaire Breen (text/editing)
Alan Whittaker (scientific review of Exploring Minerals and Crystals booklet)