Moonstones are colourless, milky white or sometimes grey or peach coloured. The prized blue adularescence or schiller - comes from deep within the stone when it is viewed at certain angles. It is not an intrinsic mineral colour. It is instead a form of iridescence arising from multi-layer interference.
Moonstone is a feldspar. Within the stone there are alternating layers of slightly different composition and these give rise to the interference colour.
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 is the first to solidify leaving an adjacent potassium rich liquid which then forms an orthoclase tending solid. That in turn leaves an adjacent albite rich liquid. And so on.. ..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. 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 in a soap bubble or oil film. 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 - see far left - coincide then there is light in that direction. When they are out of phase there is destructive interference. 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 moonstone blue because of the particular layer thicknesses and spacing.
Multilayer interference gives colours in other minerals, labradorite, opal, tiger�s eye and in the lustrous and pastel colours of sea shells.
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