Mineral properties

This is usually expressed as the main body colour of the mineral, along with the intensity of this colour. Some minerals are of differing colour along different crystal axes, a phenomenon called pleochroism (if the colour varies in two directions, the mineral is called dichroic whereas if the colour varies in three directions the mineral is called trichroic).

Some minerals are always the same colour, such as copper minerals azurite (blue) and malachite (green), but others can show a range of colours depending on different impurity elements or structural defects in their crystal structure (for example fluorite, tourmaline, quartz, corundum). For corundum, chromium causes the red colour in ruby, and iron and titanium cause the blue colour in sapphire.

This is the appearance of a mineral surface in reflected light:

• Metallic, Sub-metallic or Non-metallic
• Adamantine: the brightest, usually occurs in minerals with a high refractive index (e.g. diamond)
• Resinous (e.g. sulfur)
• Vitreous: glass-like
• Pearly: slightly iridescent
• Silky: usually in fibrous minerals
• Waxy (e.g. chalcedony)
• Earthy: very dull lustre, usually in minerals with a rough surface

Mohs' hardness scale

This is the resistance of a mineral surface to scratching. This is a simple scale of relative (not absolute) hardness ranging from 1 (talc) to 10 (diamond). In absolute hardness, corundum (9) is twice as hard as topaz (8) and diamond (10) is four times as hard as corundum. It is a very simple test requiring the scratching of the unknown mineral against a mineral of known hardness. However, minerals can be harder in different directions (e.g. kyanite is softer along its long axis (hardness of 5) than across it (hardness of 7).)

The full scale is:

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase feldspar
7. Quartz
8. Topaz
9. Corundum
10. Diamond

This relates to the number, direction and intensity of development of regular-spaced breakage directions of a mineral parallel to crystallographic planes. It reflects differing strengths of atomic bonding in different directions in the internal atomic structure. It is usually specified by its quality (perfect, good, fair, poor) and its direction. Members of the mica group have one perfect basal cleavage, feldspars have two well-developed cleavages almost at right angles. Some minerals lack cleavage (e.g. quartz, garnet) and instead have an irregular fracture surface. Pyroxenes have two distinct cleavages at 90°, amphiboles have two distinct cleavages at 120°. Galena (lead sulfide) has cubic cleavage, and fluorite (calcium fluoride) has octahedral cleavage). Parting is similar to cleavage but is breakage along planes of structural weakness such as twinning planes, caused by external stresses.

Density is the mass per unit volume but is inconvenient to measure directly. Instead, we more commonly use specific gravity, a relative density in which the mineral is weighed in air, followed by a weighing while the mineral is immersed in water (density of pure water is 1). It is a ratio between the weight of a mineral and the weight of an equal volume of water.

The specific gravity (SG) is calculated from weight in air divided by loss of weight in water, expressed in units of grams per cubic centimetre. The specific gravity of most minerals ranges from 1.5 up to 19.5.

This is the colour of the powdered form of a mineral. It is a more reliable indication of a mineral than its main body colour, as it is more constant.

It is usually tested by drawing the mineral across an unglazed porcelain tile (streak plate) to leave a coloured streak.

Transparency (diaphaneity) is the degree to which light is transmitted through a mineral. Minerals can be opaque, translucent, or transparent.

The quality of the development of crystal faces present:

• Euhedral: well-developed crystals with most crystal faces shown.
• Subhedral: Partially-developed crystals with some crystal faces shown.
• Anhedral: irregularly-formed minerals with no crystal faces shown.

This is the three-dimensional shape of an individual crystal.

General habit may be:

• equidimensional (or equant): cubic, equant polyhedral, spherical, equant anhedral.
• inequidimensional: prismatic, platy, tabular, lamellar, bladed, columnar, acicular or fibrous.
• Embayed crystals: have hollows or embayments in their outer edges.
• Skeletal crystals: have hollows or embayments in specific crystallographic orientations.
• Dendritic crystals: consist of a regular array of fibres sharing a common orientation.

Other common crystal habit terms are radiating, reticulated (e.g. criss-crossing) and botryoidal

This is the toughness of a mineral. It is the way in which a mineral is mechanically deformed:

• Fragile: easily broken by cleaving - kyanite; or fracturing- sulfur.
• Malleable: can be flattened into sheets without breaking (e.g. gold)
• Ductile: can be drawn-out into wires without breaking (e.g. copper)
• Sectile: can be cut with a blade into shavings (e.g. gypsum)
• Flexible: can be easily bent without breaking (e.g. molybdenite)
• Elastic: can be bent, and when released springs back to its original shape (e.g. micas)

Minerals can conduct electrical currents to differing degrees. Metallic minerals such as gold, silver and copper are good electrical conductors, but some (semimetals: bismuth, antimony, arsenic) are poorer conductors, termed semiconductors. Most non-metallic minerals such as silicates, carbonates and sulfates are very poor conductors of electricity.

Some non-conducting minerals can develop an electrical charge when subjected to directional mechanical stresses such as compression (piezoelectricity) or thermal stress (pyroelectricity). Quartz and tourmaline are good examples of piezoelectric minerals.

Some minerals can be strongly attracted (ferromagnetic), slightly attracted (paramagnetic) or repelled (diamagnetic) by a magnet. The most common strongly magnetic minerals are magnetite (iron oxide) and pyrrhotite (iron sulfide). Some magnetite can be a natural magnet (lodestone) and can pick up small objects such as pins, or can act as a compass needle to find north if floated on a piece of cork on water.

Some minerals containing uranium or thorium are radioactive and spontaneously emit alpha and beta particles and gamma rays that can be measured by a Geiger counter or blacken photographic film.

Some minerals emit a distinctive colour under ultraviolet light (e.g the violet glow of fluorite, the green glow of willemite, or the pink glow of manganese-bearing calcite). This is usually performed by placing the mineral in an ultraviolet viewing box with a blackened interior, in a darkened room. The colour can also vary according to whether the ultraviolet radiation is long-wave or short-wave. Fluorescence is caused by small traces of impurity elements (called activators) in the mineral’s crystal lattice.

Some minerals have a distinctive taste (e.g. salty for halite, bitter for sylvite), feel (e.g. talc feels soapy) or smell (e.g. garlic smell of heated arsenic-bearing minerals, rotten-egg smell of sulfides when they are struck by steel). It is not recommended to do this type of test outside a laboratory or on potentially toxic materials.

When a mineral grows, zoning can occur reflecting changes in the composition of the magma/fluid that the mineral is crystallising from, or from environmental changes (e.g. pH, oxidation/reduction state). This zonation is commonly along a crystallographic orientation and can be expressed in terms of a different colour, or changes in optical properties.

Fracture is the way a mineral breaks in directions other than cleavage planes, when the strength of its internal atomic bonds is about the same in all directions. Fracture can be conchoidal (curved), fibrous, splintery, hackly (jagged) or uneven.

Minerals are assigned to one of seven crystal systems, depending on their symmetry and geometrical properties. From high to low symmetry these are isometric (cubic), tetragonal, orthorhombic, hexagonal, trigonal, monoclinic and triclinic. For isometric minerals the crystal shapes can range from simple cubes (pyrite, 6 faces) to crystals showing many faces (garnet, 12 or 24 faces). Non-isometric crystals can be very complex with combinations of different shapes and many crystal faces. They can have several crystal habits ranging from long prismatic to flat tablet-like forms.

Detailed crystal structure determinations can be made by laboratory instruments such as the X-ray Diffractometer.

Minerals can be made of just one element such as copper or be a combination of several elements in a chemical compound. The compositions follow simple rules of proportion, so pyrite, iron sulfide, has one part iron to two parts of sulfur. Minerals are classifed according to their chemical groups, e.g. native elements, sulfides, carbonates, oxides, borates, tungstates, chromates, halides, sulfates, silicates etc.

Chemical composition is usually determined with complex laboratory equipment such as X-ray Fluorescence Spectrometers, Mass Spectrometers, and Scanning Electron Microscopes with spectrometer attachments. Small hand-held portable versions of some of these have been developed for more convenient laboratory and field use.