Optical and X-ray study of the low plagioclases

For each of 24 grains of low plagioclase with ‘Inferred An' contents 4.1, 6.8, 16.6, 27.7, 30.2, 35.8, 36.6, 37.6, 38.7, 43.1, 44.2, 48.0, 50.8, 51.9, 55.0, 56.5, 56.6, 57.9, 61.8, 62.1, 64.5, 76.8, 78.2, and 90.4 the following properties were measured: (1) y*, (2) location of the crystallographic axes, (3) separation of the non-Bragg e-diffractions from precession photographs, (4) refractive indices α, β, and γ, (5) the location of the optic axes, (6) the location of the principal vibration directions X, Y, and Z, (7) and a value of 2V.

Plots of y* versus the product, αβγ, which correlates linearly to anorthite content, confirm the presence of two structural discontinuities in the low plagioclase series. The angular attitude of the two optic axes and of the principal vibration directions plotted stereographically relative to b* (at S = 0, E= 0) andc (at S = 90, E = 90) for each grain follow the classic migration curves of Reinhart (1931) with slight revision. The orientation of the principal vibration axes X, Y, and Z is related quantitatively to crystallographic directions b*, c, and A by means of Euler angles Φ, θ, and ψ introduced by Burri et al. (1967) and to the crystallographic axes a, b*, and C by a set of new Euler angles θ₁, θ₂, and θ₃.