On the origin of the colour of labradorite

Abstract

Abstract Measurements have been made of the spectral distribution of the intensity of light reflected from a blue and a red labradorite for several angles of incidence over the wavelength range 3000 to 7000 Å. Transmission electron microscope observations of thin crystals of these labradorites show that the crystals consist of alternate light and dark lamellae of the order of 1000 Å thick. Although the pattern of the lamellae appears almost periodic, a statistical analysis indicates that the structure has no long‐range order. A theoretical discussion of the diffraction of light in such a structure is given and an expression for the spectral distribution of the reflected intensity derived. The spectral distributions to be expected from the blue and red labradorites have been computed using values of the mean thickness and variance of the alternate lamellae obtained from the electron micrographs. Satisfactory agreement between the observed and calculated spectral distributions is obtained. This supports an earlier suggestion that the colour of labradorite is due to optical interference in a lamellar structure.