guide to thin section microscopy - Mineralogical Society of America

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.
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