Correlation of Muscovite Sheet Mica on the Basis of Color, Apparent Optic Angle, and Absorption Spectrum

A detailed experimental study of color, apparent optic angle, and absorption spectrum (0.3 to 16μ of λ) indicates that there are basic chemical and structural differences in muscovite micas. Color identification is based upon spectrophotometer measurements. A method for quantitative evaluation of absorption band activity is defined so that single and multiple band intensities within the spectrum of an individual specimen and within spectra of different specimens can be compared. Three basic spectral types exist for the 0.3 to 1μ λ region which are comprised of various weak lines and absorption regions lying along the base of a deep absorption edge near 0.32μ. Of the three spectral types, one is associated with ruby micas, while the other two are associated with dark green and light green micas. The activities of numerous lines and bands are reported. It is shown that apparent optic axial angle is related to the position of the deep band edge except for a certain class of ruby specimens which show anomalous values. A definitive characterization of muscovite sheet is formulated in terms of the absorption coefficients at 0.44, 0.49, and 0.58μ. Direct associations are shown between certain lines in the visible λ range and certain bands in the near infrared, while the activities of a number of bands in the middle infrared are correlated to color.