Amethyst: A Symphony in Purple

Mineral Matters #370

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

Bill Mason commented on the purple strengite Mineral Matters post a couple of days ago. Bill said “I find it strange that Fe³⁺ ions can be purple rather than rusty. Think of ferric alum. I have heard that the colour of amethyst could be due to minor substitution of Fe³⁺ for Si, rather than the much touted manganese”.

So I thought I would look into it a bit more. Disclaimer: I am not an expert in this!

The formula for strengite is Fe³⁺(PO₄)·2H₂O, so the presence of Fe³⁺ is likely to be a contributing factor in its colour. But why purple? Strengite, as is the case for many minerals, can come in a range of colours, hues, and shades of colour. It is possible that there are a number of factors that may influence the colour, or the intensity of the colour.

One suggestion is that it is dependent on the amount of iron present as Fe³⁺. Another is that specific absorbed wavelengths correspond to energy levels that allow the electrons to jump to higher energy states within the iron atom.

What do you think?

Amethyst is another mineral that is said to be coloured by iron, and it can come in a range of shades from the palest purple hue through to almost black. And yes, I know that iron is not a chemical component of amethyst formula (which is simply SiO₂).

But, natural amethyst also requires exposure to ionizing radiation, such as gamma rays from radioactive elements in surrounding rocks. This radiation interacts with the iron atoms in the quartz structure. Again, the intensity of the purple colour in amethyst can vary depending on the amount of iron present. Higher iron content generally results in a deeper, more vibrant purple.

You can see a range of purple hues in the amethysts below.

Below: Amethyst, Pohndorf Mine, Toll Mountain, Jefferson Co., Montana, USA. 34mm wide.

Below: Fluorite on pinkish amethyst, Nasik, India. 80mm wide.

Below: Pinkish-purple amethyst, Baobab Mine, Kitui District, Kenya. 70x50x40mm. 

Below: Very pale amethyst, but very dark at the base of the crystals, Wave Hill, Northern Territory, Australia. Width ~60mm.

Below: Amethyst so dark that it looks black unless backlit, Wyloo, Western Australia. ~110mm tall.

Below: Strong purple amethyst with darker tips, Victoria River Downs Station, Northern Territory, Australia. ~35mm wide.