Halite with blue zones

Halite from Grube Merkers in Thüringen. This is clear NaCl with embedded blue zones. The two spectra show the transmission of a clear (orange line) and a blue region (blue line). These were obtained by shining a fibre source into the crystal mass at points that did not and did illuminate a blue region. The 'clear' spectrum is probably somewhat contaminated by some blue light scattered inside the sample.

The left image shows the crystal mass illuminated from the front and side by white (6500K) LED lamps. The right image is with a fibre probe illuminating the rear of the sample. This is an approximately 3000K filament lamp.

Coming mostly - but not exclusively - from Germany in deposits that are associated with potassium minerals, gamma radiation from the radioactive potassium-40 isotope irradiates the halite. This results in the creation of colour (F-)centers which are mostly due to electrons trapped at the sites of missing Cl- ions. This turns the crystals amber in colour. Subsequent pressure or crystal deformations, followed by illumination by visible light, converts the colour to blue (see: worldtracker.org/media/library/Science/Science%20Magazine... ). According to minerals.gps.caltech.edu/color_causes/Radiate/index.html , the migration of these F-centre electrons to sodium ions (Na+) leaves sodium metal atoms that gradually aggregate to a colloidal form which causes the blue colour.

This sample was kindly provided by Dr Tim Otto Roth.

Added on 4 October 2014

So here is the process, in a nutshell, as far as I understand it. I am ready to modify the ideas as I learn more!

The halite is irradiated by gammas from the potassium-40 isotope that lives as neighbour to the salt.

The gammas create F-centres: electons trapped at Cl- vacancies. These make the halite appear yellow since the F-centres absorb 400-500nm light.

Processes, including local strains in the crystal, cause the F-centres to coalesce to form a series of other colour centres with increasing numbers of electrons: R1-(F3), R2-(F3), M-band...

At some point, the trapped electrons become associated with clusters of Na+ ions to create a neutral metal nanoparticle.

The electrons lie in the conduction band of a sodium colloid particle of nm dimensions. Light covering a relatively broad absorption band can excite plasmons by causing the electron gas in the colloid particle create a quantized oscillation (related to Mie scattering).

It is this absorption band - lying at around 600nm - that makes the halite appear blue in transmission. The exact position of the absorption band depends on the size of the colloid particle - hence the observed range from 'navy' blue to purple.

The colour can be destroyed by dissolving the salt or by recrystallising it.

This is one of the more subtle colouring mechanisms but is, I assume, very common - especially in ancient stained-glass windows using colloidal gold...

Such colours can be brilliant and stable.

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