guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Colour and pleochroism Figure 4-14 A-C. Change of absorption colour in crystal sections of biotite, actinolite and aegirine-augite as the stage is rotated 360°. Shown are the four positions in which the vibration directions of the two waves coincide exactly with the directions of the polarizers. In these orientations only the E-W vibrating wave passes the crystal; the N-S wave is not activated. Therefore, these crystal sections change their colour every 90° of rotation. In the actinolite and aegirine-augite sections, these are the colours relating to the n y and n x’ waves, and in biotite, the colours relating to the n z~y and n x waves. As far as clinoamphiboles are concerned, the absorption colours parallel to X, Y and Z are determined in two specific sections (Figs. 4-15,16,17): (1) In crystal sections parallel to (010), the vibration directions Z and X are in the viewing plane, except for some rare alkaline amphiboles. Such sections are commonly prismatic in shape and can be recognised by their high interference colours (∆n = n z -n x ) (see Ch. 4.2.3). Raith, Raase & Reinhardt – February 2012 (2) In crystal sections perpendicular to c, the vibration directions Y (parallel to b) and X’ are in the viewing plane. These crystal sections are recognised by the characteristic intersection of the {110} cleavage planes. Crystal sections perpendicular to one of the two optic axes appear in a single colour corresponding to Y as the stage is turned (n y wave only). 73
Guide to Thin Section Microscopy Colour and pleochroism Figure 4-15. Determination of the colours for the principal vibration directions Z, Y and X of an optically biaxial mineral, using actinolite, Ca 2 (Mg,Fe 2+ ) 5 [Si 8 O 22 ](OH) 2 , as an example. Raith, Raase & Reinhardt – February 2012 74
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