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Why are Minerals coloured?

The mineral colouring is one of the primary physical properties of a mineral that gives away a lot about the mineral. Mineralogy considers colour to be the foremost property of a natural compound that diagnoses the quality of the gem and its characteristics.

Recognising the existence of colour in minerals began with pre-historic ancestors of our who coloured the paintings they drew on caves with iron oxides and charcoal. These paintings continue to retain the intensity of colour to this day.

The purpose of studying a mineral’s colour is that it betrays the genetic and chemical composition of minerals. It is also used to synthesise the analogues of naturally occurring gems.

Difference between ores and crystals

When it comes to the colour of minerals in Lumps of ore and individual crystals, the colour that the minerals in thin transparent section project under a microscope, the colour that the minerals project in sections that are polished under a light reflected at them and the colour that is visible when the mineral is reduced to a fine powder, i.e., the mineral’s streak; a basic demarcation exists.

There are certain existing benchmarks that are used in order to evidently describe the colour of the mineral. This involves contrasting the colour of the mineral to a substance or colour that is already prevalent in everyday use, like sea green, parrot green, apple red, and so on. They are also often compared to the colour standards of minerals themselves, like emerald green or vermillion red.

When it comes to ore minerals, the colours that are projected by alloys and metals are used to describe them. Some examples can be, copper-red or native copper, steel grey or molybdenite, brass yellow or chalcopyrite, and so on. Although incomplete at this point in time, there are people working towards the techniques that will be helpful in utilising basic calorimetric characteristics to objectively evaluate the colour of minerals, with special consideration towards gems.

A majority if minerals tend to appear to be of different colours depending on their crystallographic directions. A few even go to the extent of entirely changing their colour when exposed to the colour temperature of their radiative source.

Mineral groups based on Mineral Colouring and their properties

Depending on the colour’s properties, there are three main groups of minerals that have been identified. These are:

Idiochromatic Minerals

The colour in the minerals exists because of the chemical element, that are the impurities of elements that form species. These act as chromophores. Other responsible factors are the defects that are present in the crystal, like its vacancies and the zonal structure of the crystal. The type of optical absorption also influences the difference between the various subgroups of idiochromatic minerals.

The colour that is caused due to radioactivity is not far from the electron-hole colour centres formed by the natural ionizing radiation.

Allochromatic Minerals

The colour in these minerals is caused by the existence of impurities that are often other coloured minerals but can also sometimes be the bubbles that are formed by gases or liquids. For instance, parse, a type of quartz is green in colour because of inclusions of chlorite or actinolite.

Pseudoachromatic Minerals

The colour in these minerals appears with the light interference and diffraction, along with the incident white light’s internal reflection, refraction and dispersion. A mineral formation’s structural features and the crystal’s surface layers (different types of tarnish, like covellite, chalcopyrite and bornite’s iridescent film) dictate these phenomena. The phenomena are in conjunction with the mineral formation’s structural features and the crystal’s surface layer.