October 2018

Identifying Minerals

Geoscience Australia

The term mineral is used to describe a naturally occurring solid substance with a reasonably fixed chemical composition and crystal structure. 

Topaz, From the collection of: Geoscience Australia

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.

Rhodochrosite, From the collection of: Geoscience Australia

Many mineral names end in ‘ite’. This suffix is derived from the Greek word lithos (from its adjectival form -ites), meaning rock or stone.

Peridot, From the collection of: Geoscience Australia

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.

Varieties of quartz, From the collection of: Geoscience Australia
Colour
The colour of minerals can be deceiving. Minerals with the same chemistry can have different colours, usually caused by minor impurities in the crystal (such as trace amounts of iron, titanium or manganese). Quartz can be clear, white, pink, purple, grey, red, yellow, green, brown and even black. 
Ruby in host rock, From the collection of: Geoscience Australia

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.

Corundum, From the collection of: Geoscience Australia

When corundum contains iron and titanium it is blue and is called a sapphire.

Limonite streak, From the collection of: Geoscience Australia
Streak
Scientists use the streak rather than the colour of a mineral as a more reliable identification method. The streak is the colour of the powdered mineral and is often found by scratching the sample on an unglazed white porcelain tile (streak plate). Even with the presence of impurities, the colour of the streak remains consistent. 
Pyrite streak, From the collection of: Geoscience Australia

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.

Malachite streak, From the collection of: Geoscience Australia

The streak of malachite is green.

Galena streak, From the collection of: Geoscience Australia

Galena has a steel grey streak.

Sphalerite streak, From the collection of: Geoscience Australia

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.

Hematite streak, From the collection of: Geoscience Australia

The streak of hematite is red to reddish brown.

Gypsum (Satin Spar), From the collection of: Geoscience Australia
Lustre 
Lustre or sheen describes how light is reflected from a mineral’s surface. It indicates what the mineral surface looks like, disregarding its colour. The two basic divisions of lustre are metallic and non-metallic. A number of other words are often used to describe non-metallic lustre – glassy, earthy, pearly, greasy, dull, adamantine (diamond-like), silky and resinous. This gypsum has a silky lustre.
Galena lustre, From the collection of: Geoscience Australia

Galena has a metallic lustre.

Sphalerite lustre, From the collection of: Geoscience Australia

Sphalerite has a resinous lustre.

Quartz lustre, From the collection of: Geoscience Australia

Quartz has a vitreous lustre. This means it looks like broken glass.

Muscovite mica lustre, From the collection of: Geoscience Australia

Muscovite mica has a pearly lustre.

Talc lustre, From the collection of: Geoscience Australia

Talc has an earthy lustre which is dull with no shine.

Diamond, From the collection of: Royal Ontario Museum
Hardness
Minerals can also be identified by comparing their relative hardness. If a substance is able to scratch another substance, it is harder. A standard hardness scale, called Mohs Scale of Hardness, is named after the scientist Friedrich Mohs (1773-1839). It is a relational scale that ranges from 1 for the least hard minerals to 10 for the hardest mineral. For example, quartz with a hardness of 7 will scratch feldspar which has a hardness of 6. Diamond is by far the hardest mineral and will scratch every other mineral. Some common materials of known hardness are often used to determine the relative hardness of a mineral e.g. a fingernail (2.5) will scratch gypsum; steel (6.5) in a knife should scratch feldspar but not quartz.
Mineral hardness diagram, From the collection of: Geoscience Australia
Kyanite, From the collection of: Geoscience Australia

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.

Diamond atomic structure, From the collection of: Geoscience Australia
Crystal Structure
Atoms within crystals are arranged in a lattice which has a regular internal framework. There are six fundamental ways atoms in crystals can be arranged, forming the six different crystal systems. The systems are based on symmetries within the lattice. The crystal structure influences properties like hardness, crystal shape and cleavage. 
Uncut diamond, From the collection of: Geoscience Australia
Crystal Shape
The shape that a mineral will take as it crystallises reflects the internal arrangement of its atoms. Some crystal shapes are particularly recognisable, like this octahedral diamond. Many perfect crystal shapes can be described as geometric shapes.
Ruby, From the collection of: Geoscience Australia

Hexagonal outline of ruby crystal.

Almandine, From the collection of: Geoscience Australia

Dodecahedral almandine (type of garnet) crystal.

Pyrite cube, From the collection of: Geoscience Australia

Cubic pyrite crystal. Iron pyrite (FeS2) is often found as cubic crystals known by old time miners in Western Australia as “Devil’s dice”.

Fluorite, From the collection of: Geoscience Australia

Octahedral fluorite crystal.

Muscovite mica, From the collection of: Geoscience Australia
Cleavage
When crystals break, they split along straight faces called cleavage planes which are weak due to the atomic structure of the crystal. Mica has one cleavage plane which allows it to break into flat sheets.
Diagram that describes mineral cleavage., From the collection of: Geoscience Australia
Calcite, From the collection of: Geoscience Australia

Calcite (CaCO3) has regular cleavage at 60° and 120° resulting in beautiful rhombic crystals.

Halite, From the collection of: Geoscience Australia

Halite (rock salt) has cleavage in three directions, resulting in cubic crystals.

Quartz, From the collection of: Geoscience Australia

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.

Cerussite, From the collection of: Geoscience Australia
Twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in specific configurations. 
Calcite, From the collection of: Geoscience Australia

Fishtail twinned calcite.

Diamond, From the collection of: Geoscience Australia
Density
Density is the mass per unit volume and can be calculated precisely by measurement of mass and volume. Precious stones such as diamond, zircon and rubies are easily distinguished by differences in density.
Gold nugget, From the collection of: Geoscience Australia
Specific Gravity
Specific gravity (relative density) relates the measured density of a material to a reference substance, usually water. One method to determine specific gravity is to weigh a sample in air followed by a weighing while the mineral is immersed in water. The specific gravity is calculated by dividing the weight in air by loss of weight in water. Water has a specific gravity of 1. Gold has a specific gravity of about 19. This means that a certain volume of gold weighs 19 times more than the same volume of water. The specific gravity of most minerals ranges from 1.5 to 19.5. If a mineral floats on water, its specific gravity is less than 1. 
Galena, From the collection of: Geoscience Australia

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.

Mesolite, From the collection of: Geoscience Australia
Crystal Habit or Form
The habit or form of a crystal refers to the characteristic shape in which a mineral commonly grows. Most minerals are not perfectly formed, individual crystals. They are aggregates of crystals which often have a distinct appearance such as radiating like a fan, rounded little balls or needle-like masses. Any particular mineral may display more than one habit. This mesolite has an acicular habit, it occurs as needle-like crystals.
Natrolite, From the collection of: Geoscience Australia

This specimen of natrolite displays an acicular habit: needle-like, slender and/or tapered crystals.

Rhodonite, From the collection of: Geoscience Australia

Rhodonite with a columnar habit: long, slender prisms often with parallel growth.

Biotite mica, From the collection of: Geoscience Australia

Biotite mica displaying a foliated or lamellar habit: layered structure, parting into thin sheets.

Mimetite, From the collection of: Geoscience Australia

This specimen of mimetite has a globular or botryoidal habit: grape-like, hemispherical masses.

Tourmaline, From the collection of: Geoscience Australia

These crystals of tourmaline have a prismatic habit: elongate, prism-like crystals.

Gypsum (Desert Rose), From the collection of: Geoscience Australia

Desert rose has a rosette or lenticular habit: platy, radiating rose-like aggregate.

Inesite, From the collection of: Geoscience Australia

This inesite has a stellate habit: star-like, radiating crystals.

Quartz, From the collection of: Geoscience Australia
Transparency 
If the outline of an object can be seen clearly through a mineral then that mineral is transparent. If a mineral transmits light but objects cannot be seen clearly then that mineral is said to be translucent. A mineral that does not transmit light is termed opaque. Quartz can range from transparent (as in this sample) to opaque.
Amethyst, From the collection of: Geoscience Australia

This sample of amethyst is translucent.

Malachite, From the collection of: Geoscience Australia

Malachite is opaque.

Banded Iron Formation with magnetite rich layers, From the collection of: Geoscience Australia
Magnetism
Magnetism is a distinctive property in a small number of minerals. Magnetite is the prime example, but a few other minerals may weakly attract a magnet, notably chromite (a black oxide) and pyrrhotite (a bronze sulphide). Large iron-bearing rock masses may affect the orientation of compass needles.
Calcite effervescence, From the collection of: Geoscience Australia
Effervescence
Effervescence (fizz) can be produced when a drop of dilute (5% to 10%) hydrochloric acid is placed on a rock or mineral. Through this acid testing bubbles of carbon dioxide gas can released. The bubbles signal the presence of carbonate minerals such as calcite or dolomite. The bubbling release of carbon dioxide gas can be so weak that you need a hand lens to observe single bubbles slowly growing within the drop of hydrochloric acid - or so vigorous that a flash of effervescence is produced. These variations in effervescence vigour are a result of the type of carbonate minerals present, the amount of carbonate present, the particle size of the carbonate, and the temperature of the acid.
Graphite, From the collection of: Geoscience Australia
Conductivity
Minerals can conduct electrical currents to differing degrees; metallic elements are good conductors whereas silicates are very poor conductors. Some non-conducting minerals can conduct electrical currents well when they are subjected to directional mechanical stresses such as compression (piezoelectricity) or thermal stresses (pyroelectricity). Quartz and tourmaline both have this property. Graphite is a good conductor of electricity but this property is not tested in the field.
Pyrite conductivity, From the collection of: Geoscience Australia

Classroom experiment testing electrical conductivity of pyrite - the bulb illuminates.

Apophyllite under UV light, From the collection of: Geoscience Australia
Fluorescence 
Some minerals emit a distinctive colour under ultraviolet light (e.g. the blue or green glow of fluorite, the green glow of willemite, or the pink glow of manganese-bearing calcite). The colour emitted can vary according to whether the ultraviolet radiation is long-wave or short-wave.
Langbanite under UV light, From the collection of: Geoscience Australia
Willemite, Franklinite and Calcite under UV light, From the collection of: Geoscience Australia
Credits: Story

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)

Credits: All media
The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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