guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Double refraction Interference colour and zoning of minerals: In minerals with extensive solid solution, the optical properties vary with chemical composition, including birefringence. Chemical zoning in crystals can then be recognised from the conspicuous zoning of the interference colour. Particularly impressive examples are plagioclases and titanaugites with oscillatory zoning, as found in certain volcanic rocks, zoned melilite in slags, and epidote in metamorphic rocks (Fig. 4-34). Figure 4-34. Zoning of interference colour in plagioclase (A), titanaugite (B), sodic amphibole (C) and epidote (D). Growth zoning with oscillatory variations in composition characterises the volcanic plagioclase and augite. The sodic amphibole has a Fe 3+ -rich core (crossite) and a Fe 3+ -poor rim (glaucophane). The decrease of interference colours towards the edge of the epidote grain indicates decreasing Fe 3+ contents during grain growth. The interference colours of coloured minerals may be masked significantly as they overlap with the minerals' own colour (examples: hornblende, biotite). In strongly coloured minerals of high birefringence, the primary mineral colour will be dominant under crossed polarizers. A relatively pale interference colour or a high-order white can hardly impose a noticeable spectral shift on the mineral's own colour as the two overlie each other (example: rutile). Raith, Raase & Reinhardt – February 2012 Anomalous interference colours: The interference colours of certain minerals deviate from the normal colour scheme. Instead of the grey to white colours in the first order of the interference colour spectrum, leather brown, ink blue to grey-blue colours are observed (Fig. 4-35,36). Such "anomalous" interference colours are generated by a strong dispersion of birefringence, which means that the latter attains distinctly different values for different wavelengths (colours). In the melilite example, the value of birefringence becomes zero for wavelengths in the range orange-yellow-green. Hence, these colours are not contributing to the interference colour (cf. Fig. 4-28). Figure 4-35. Examples of anomalous interference colours: A. Titanaugite (sector structure and oscillatory zoning); B. Clinozoisite; C. Zoisite; D. Vesuvianite; E. Melilite (continuous zoning with åkermanite-rich core and gehlenite-rich rim). B C D 97
Guide to Thin Section Microscopy Double refraction Raith, Raase & Reinhardt – February 2012 Figure 4-36. Anomalous interference colours in chlorite and melilite. Anomalous blue or leather-brown interference colours occur in solid solutions with birefringence values close to zero. The higher birefringent members of the solid solution series show normal light grey to creamy white interference colours of the first order. 98
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guide to thin section microscopy - Mineralogical Society of America

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