Identifying Minerals

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. 

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. 

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.

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.

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. 

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.

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. 

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.

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.

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.

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.

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.

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.