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Tuesday, 4 May 2021

Of quartz – A colourful problem

Where does colour come from? Pigments, we say: the ability of certain materials to absorb and reflect different colours of light as electrons are excited along or within their structure. It’s true: sometimes we have to go as deep down as the chemistry, the connectivity between atoms, to see where colour comes from. Other times we don’t.

Structural colour arises from the nanostructure of materials: the arrangement of atoms in giant structures, or of groups of molecules. As they cluster together, they form planes, angles, surface details, and other interesting geometry with gaps and overlaps the same size as the wavelength of light. Like the colours seen on an oil slick, this structural colour happens when light interacts with the shape of matter.

The morpho butterfly has wings with structural colour. Notafly via Wikipedia Commons.

The most famous example of structural colour is the morpho butterfly. Microscopic fern-like structures present on its wings make it appear an iridescent blue.

Emerald. Parent Géry.
Ruby. Parent Géry

Metal impurities present in minerals can disrupt their regular lattices and confer colour. For instance, chromium is responsible for the red colour of ruby – and the green colour of emerald. How can that be? The answer is that it’s not because chromium is inherently red or green. Colour depends upon the matrix of the mineral it's embedded in as well as the metal and the way it bonds to this matrix. Colour is unpredictable. Or nearly unpredictable.

Structure of emerald. Apollo240.
Structure of ruby. Gavin Whittaker, Miramodus ltd


One rather mysterious mineral is the common one, rose quartz. It’s based on the quartz structure, but ordinary quartz is white, and rose quartz is pink. Why? Scientists just don’t know. It could be structural or it could be due to small amounts of mineral impurities. Small amount of titanium and manganese have been found in rose quartz, but scientists have been unable to identify concentrations that reliably give rise to the colour, and attempts to make it artificially have failed. So maybe it’s not that. An alternative theory is that fibrous mineral structure within the crystal gives it that pink – but this has not been solved or replicated either.
Rose quartz. Piotr Sosnowski

So, for the meantime, this common mineral remains as a colourful problem, awaiting a solution!

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