Little school of mineralogy 3
Why do minerals have different lustres?
The lustre of a mineral depends on the reflection of light rays, and it is more intense in minerals with a higher refractive index and smooth, flat crystal faces.
Mineral lustre can be adamantine, metallic, vitreous, silky, pearly, greasy, resinous, waxy or earthy.
What is mineral cleavage?
Minerals often have different cohesion, the forces by which particles in the crystal structure are connected, in different directions, so when an external force acts on a mineral, regular splitting occurs parallel to existing or possible crystal faces.
This property of minerals is called cleavage.
Cleavage is visible on a mineral as a system of parallel lines transverse to a particular face.
Some minerals have no cleavage, while others may have one or more systems of perfect, good, imperfect, indistinct or poorly expressed cleavage.
What is mineral fracture?
Unlike cleavage, fracture is the irregular breaking of a mineral due to the action of force, and it is expressed in minerals where cohesion is approximately equal in different directions.
Fracture can be hackly, splintery, uneven or conchoidal.
Quartz is an example of a mineral without cleavage in which conchoidal fracture is expressed.
What is mineral hardness?
Hardness is the resistance of a mineral to the action of mechanical force, for example indentation, scratching, grinding and similar actions.
It is another mineral property related to cohesion.
There are examples of minerals having different hardness in different directions, such as kyanite, whose synonym disthene refers precisely to this property.
How is mineral hardness expressed?
In mineralogy, mineral hardness is most often expressed as relative hardness, R, according to the Mohs hardness scale – a sequence of ten minerals arranged from the softest mineral, talc, to the hardest mineral, diamond.
The Mohs scale is not linear, which means that the hardness of diamond is not ten times greater than the hardness of talc, but according to absolute hardness, A, diamond is much harder.