| Moonstones are colourless, milky white or sometimes grey or peach coloured. The prized blue - adularescence or schiller - comes from deep within the stone when viewed at certain angles. The blue is not an intrinsic mineral colour. It is instead a form of iridescence arising from multi-layer interference of light.
Moonstone is a feldspar, within the stone there are domains of alternating layers of slightly different composition.
Feldspars are complex sodium potassium aluminium silicates. Albite (sodium rich) and orthoclase (potassium rich) are compositional extremes or end members of feldspars. As a feldspar melt cools, albite solidifies first leaving an adjacent potassium rich liquid which then forms an orthoclase tending solid. That leaves an adjacent albite rich liquid. The outcome can be a regular stack of alternating layers each less than a micron thick of sodium rich and potassium rich material.
The alternating layers have slightly different optical properties and light passing through them is reflected and refracted at their interfaces. The reflected light waves interfere to produce the adularescence colours.
The interference mechanism is more easily understood by looking at what happens at a single thin layer as is found in a soap bubble or an oil film on water. Light striking the layer is part reflected from the front surface. Another part enters the film, reflects off the rear surface and leaves in the same direction as the front reflected wave. The two outgoing waves combine. If their wave crests (shown at far left as red for positive and blue for negative) coincide then there is light in that direction. When they are out of phase there is destructive interference and there is darkness. The phase condition is wavelength dependent with the result that we see interference colours.
The effect is much enhanced when there are multiple layers of regular spacing and thickness. The light leaving the structure is then the outcome of several reflections between layers, each with interference possibilities. We see blue because of the particular layer thicknesses and spacing in moonstone.
Multilayer interference gives colours in other minerals, labradorite, opal, tiger’s eye and in the lustre and pastel colours of sea shells.
|Alternating layers of feldspars of slightly different refractive index produce multilayer interference and a moonstone's blue light.|
Single layer interference as in soap bubbles and oil films.