Strengite, Indian Mountain, Alabama

Mineral Matters #367

This week’s Facebook Mineral of the Week Group’s selected theme is purple minerals.

Purple is a fascinating colour that bridges the gap between the warm reds and the cool blues on the light spectrum. All colours we perceive arise from the interaction of light with our eyes. Visible light is a form of electromagnetic radiation with a range of wavelengths, and purple corresponds to wavelengths between roughly 380 and 450 nanometres (nm).

As some of you know, I am interested in colour due to my interest in art. Although I don’t grind up my specimens to make pigments! 😏

Natural sources of pure purple pigments were scarce in ancient times. One source, Tyrian purple, was derived from a specific sea snail and was incredibly expensive. This rarity made purple a symbol of royalty and wealth in many civilisations.

I wonder whether purple strengite would make a good pigment. I suppose it depends on what causes the colour.

The colours we see in minerals arise from an interplay between the mineral's composition and how it interacts with light. This may include things like:

• The presence of specific elements.

• Metal ions and charge transfer. Minerals often contain metal ions in various oxidation states (e.g., iron existing as Fe2+ or Fe3+). These ions can absorb specific wavelengths of light depending on their electronic configuration.

• Defects and irregularities. Imperfections within a mineral's crystal structure, such as vacancies or dislocations, can also act as light filters. These defects can absorb specific wavelengths.

• Impurities and trace elements. Even tiny amounts of foreign elements within a mineral's crystal structure can significantly impact its colour. Manganese, for example, can cause a range of purples, pinks, and browns depending on its oxidation state.

Below: Strengite, Indian Mountain, Cherokee Co., Alabama, USA. Width of view 3mm.