guide to thin section microscopy - Mineralogical Society of America

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Guide to Thin Section Microscopy Extinction The colour shift to a lower order is not always easily identified in minerals with high birefringence and interference colours of the 3 rd or higher order, particularly for the novice. In such cases, the quartz wedge may be used instead of the first-order red plate. By inserting the quartz wedge, the continuous change of the interference colours towards more saturated hues can be observed while the colours shift to lower orders. In minerals with low birefringence first-order black is reached initially. With further insertion of the quartz wedge, the colour sequence reverts to increasing interference colours (e.g., olivine and white mica). At thinningout crystal plate edges, the colour bands move towards the grain interior as the quartz wedge is inserted (Fig. 4-44). This effect is particularly evident from the red tones of the first, second and third order. The dark grey colour band of the first order at the outermost edge of the mineral migrates into the grain interior and is replaced by colours of the first to third order, if the interference colours of the mineral lie in the first to second order (e.g., olivine, white mica, pyroxenes). Interpretation: In an anisotropic mineral, the n x ' wave advances faster in the crystal plate than the n z ' wave. Both waves have different wavelengths. After exciting the mineral, both waves have the same velocity and wavelength, but with an accumulated retardation of Γ Min = d * (n z ' – n x '). With this retardation, the waves enter the crystal plate in the compensator in which the fast and slow wave directions are at 90˚ to those of the mineral. The mineral's n x ' wave now becomes the slower wave n zComp , the original n z ' wave transforms to the faster wave n xComp . Therefore, the retardation accumulated in the mineral is now reduced by the retardation of the compensator. There is a decrease of interference colours: Γ Min – Γ Comp = Γ total . When inserting the first-order red plate or the quartz wedge, the following is to be noted: 1. If the retardation of the mineral is larger than that of the compensator, only decreasing colours of a lower order are observed. 2. If the retardation of the mineral is the same as that of the compensator, the mineral appears in first-order black (Γ total = 0). The retardation of the mineral is fully compensated. Raith, Raase & Reinhardt – February 2012 3. If the retardation of the mineral is smaller than that of the compensator, the interference colour are reduced to a value corresponding to the difference Γ Comp – Γ Min . When inserting the quartz wedge, decreasing colours will be observed initially, down to first-order black, after which colours will increase again. 107
Guide to Thin Section Microscopy Extinction Figure 4-44. Anhydrite grain showing a colour sequence covering seven orders. Raith, Raase & Reinhardt – February 2012 Addition and subtraction can be easily recognised by observing the changes in the colour sequence along the wedge-shaped edge of the grain. Compensation with first-order red shifts the colour sequence into the grain interior by one order (subtraction), or towards the grain edge by one order (addition). When using the quartz wedge for compensation, the colour sequence moves either into the grain interior by four orders (subtraction) or out of the grain (addition). The interference colours of minerals that are strongly coloured in plane-polarized light may be masked significantly such that addition and subtraction positions with inserted frist-order red plate are not easily distinguished by a novice. In such cases, it is recommended to use the quartz wedge and to observe the movement of colour bands at the wedging-out edges of the mineral grains. 108
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guide to thin section microscopy - Mineralogical Society of America

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