Properties of minerals used in identification
Minerals can be identified using a number of properties. These include physical and chemical properties such as hardness, density, cleavage and colour, crystallography, electrical conductivity, magnetism, radioactivity and fluorescence.
Hardness: Mohs' hardness scale
This is a simple scale of relative (not absolute) hardness ranging from 1 (talc) to 10 (diamond). 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:
- Orthoclase feldspar
Density and Specific Gravity
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).
The specific gravity (SG) is calculated from weight in air divided by loss of weight in water. The specific gravity of most minerals ranges from 1.5 up to 19.5.
This relates to the number and intensity of development of regular-spaced breakage directions of a mineral parallel to crystallographic planes. 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°. When a cleavage is poorly developed it is called a Parting.
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)
This is the degree to which light is reflected by the surface of a mineral:
- 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
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.
Minerals are transparent when their atoms are packed differently in different directions and hence the velocities of light through the mineral also differ in different directions.
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 (e.g. globular).
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 (e.g. 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 are good examples of this.
Some minerals can be strongly attracted (ferromagnetic), slightly attracted (paramagnetic) or repelled (diamagnetic) by magnetic forces. The most common strongly magnetic minerals are magnetite and pyrrhotite.
Some minerals (particularly those containing uranium) 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.
Taste, touch and smell (not usually performed)
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.
Ondine Evans , Web Researcher/Editor