Colour is usually the first thing that people notice about a mineral, however, it is not usually a diagnostic property since many minerals can have the same colour and a single mineral can exhibit a wide range of colours. For example, quartz may be pink, black, yellow, purple or colourless.
The sample of pyrite shown in this image would be described as gold in colour.
Hardness is a measure of a mineral’s resistance to scratching. Hardness is usually measured on the Moh’s Hardness Scale which ranks a mineral on a scale of 1 (softest – ie. Talc) to 10 (hardest – ie. Diamond).
The sample of biotite shown in this image is quite soft and has a Moh’s hardness of 2.5-3.
Lustre is the quality and intensity of light that is reflected from the surface of a mineral. Lustre is divided into two categories: Metallic and Non-Metallic.
Metallic Lustre: A mineral with a metallic lustre appears as though it is made of metal.
Non-Metallic Lustre: A mineral with a non-metallic lustre has several subtypes:
- Vitreous/glassy – glazed appearance like glass
- Earthy – resembles the surface of unglazed pottery
- Silky – has the appearance of silk or satin
- Pearly – iridescent appearance, like pearls
The sample of calcite shown in this image would be described as having a vitreous lustre.
Cleavage and Fracture
Cleavage is the ability of a mineral to break along preferred planar directions. This occurs because of a weakness between atoms along these planes. Since the atomic structure of a mineral is unique, cleavage can be used as a diagnostic tool.
Cleavage can be described by the number of planes, the angles at which cleavage planes intersect, and the quality of the cleavage present.
The sample of halite shown in this image has 3 planes of cleavage that intersect at 90°.
Fracture is the way a mineral breaks when not controlled by cleavage. Minerals without cleavage commonly have an irregular fracture. Quartz exhibits a conchoidal fracture.
Streak is the colour that is visible when a mineral is scratched along the surface of unglazed porcelain – known as a streak plate. The streak colour is a more reliable method of identification than looking at the colour of the hand sample.
The sample of hematite shown in this image leaves a reddish-brown streak on a streak plate, even though the hand sample colour may range from black, silver to reddish-brown.
Crystal Habit refers to the external shape/geometry of a mineral crystal if it were to crystallize freely. These crystals are described as euhedral. When a mineral does not display a definite crystal form it is described as anhedral.
There are many possible crystal habits. These include hexagonal pyramid, blocky, hexagonal, cubic, rhombohedral, dodecahedron, prismatic and bladed.
The sample of quartz shown in this image has a hexagonal pyramid crystal habit.