Little school of mineralogy 2
How do we explain the colour of minerals?
The colour of a mineral is, along with its shape, the first feature noticed on a mineral.
It reflects the nature of the interaction between electromagnetic radiation of the visible spectrum and the electrons of the atoms, ions or molecules from which a particular mineral is built.
The colour of a mineral is the result of selective absorption and reflection of light rays from the surface of the mineral.
What are idiochromatic minerals?
Idiochromatic minerals have their own characteristic colour because, as essential components, they contain elements that absorb a certain part of the visible light spectrum.
Their colour is a constant and predictable property of the mineral.
Examples of idiochromatic minerals are azurite, which is always blue, cinnabar, which is red, and malachite, which is green.
What are allochromatic minerals?
Allochromatic minerals are minerals whose colour is caused by inclusions of other minerals, the presence of trace elements or defects in the structure, which is most often variable.
Their colour is a variable and unpredictable property of the mineral.
An example of an allochromatic mineral is quartz, which occurs in nature in differently coloured varieties such as purple amethyst, yellow citrine, brown smoky quartz, black morion, rose quartz and others.
What are pseudochromatic minerals?
Pseudochromatic minerals have an apparent colour caused by changes on or below their surface due to the diffraction of light.
Their colour is variable, but this variability is a unique property of the individual mineral.
The best-known examples are opal, which has a unique “play of colour”, and labradorite, which has unique light reflections known as the “schiller effect”.
What is the streak of a mineral?
The streak of a mineral is the powdery trace left by a mineral when it is rubbed across a rough ceramic surface, and it corresponds to the true colour of the mineral.
Since colour is one of the primary diagnostic properties of minerals in mineralogy, mineralogists also determine the streak of a mineral in order to identify the true colour of a particular mineral.