Labradorescence and the ideal behavior of thicknesses of alternate lamellae in the Bφggild intergrowth

Abstract

The well-known optical behavior of iridescent labradorite is discussed. The observation of labradorescence, in which the color changes from violet to red as the lamellar thickness and An content increase, is supported by the results of calculation. Theoretical treatment of the optical phenomenon of labradorescence corresponds to a kinematic theory of diffraction. Based on the observations with an electron probe microanalyzer and an electron microscope, the ideal behavior of alternate lamellae in the Bφggild intergrowth is estimated. The thickness of An-rich lamella increases up to An_65±, while that of An-poor lamella increases up to An₅₀ and then decreases describing a parabola.
The regression equation for the lamellar thickness d(Å) and the wavelength λ(Å), where the maximum of reflected intensity appears, is obtained as follows : λ=3.105•(d)-21.178. The result of calculation from the equation obtained suggests, together with this regression equation itself, that the maximum of reflected intensity takes higher value when (n_a, n_b)=(1.55, 1.57) in violet- to blue-iridescent labradorite, (1.55, 1.56) in green one, (1.54, 1.57) and (1.55, 1.56) in yellow-to red-one, where n_a=(α_a+β_a+γ_a)/3 and n_b=(α_b+β_b+γ_b)/3, than the case of other combinations. This calculated result may sufficiently explain the experimental results and data obtained from analytical electron microscope.