guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Double refraction
are "filtered out", while red and blue hues dominate. The range Γ = 500 ↔ 1500 nm is
characterized by relatively intense colours due to elimination or reduction of one or two
relatively narrow spectral intervals for each Γ value. For high retardation values (Γ > 1500
nm) an increasing number of spectral domains distributed over the full wavelength spectrum
is subtracted which produces increasingly pale colour tones and, at very high Γ, an
interference colour that is perceived as white. As can be derived from Fig. 4-27, at very high
retardation the interference colour spectrum consists of a narrow-spaced and evenly
distributed arrangement of domains of constructive and destructive interference, such that
none of the major colour bands is cut out completely.
Thus, the interference colour spectrum starts with black (Γ = 0) and progresses through grey,
white, yellow, and orange to a succession of intense colours of red → blue → green → yellow
→ orange → red, which repeats itself with increasing retardation, while getting more and
more pale (Fig. 4-29; Michel-Lévy colour chart). The colour sequence is subdivided into
colour orders using the distinctive purplish reds (in steps of 551 nm). From the 4 th order
upwards, the interference colours are dominated by alternating greenish and reddish hues.
With increasing retardation these fade more and more and eventually approach white (Fig. 4-
33). This is referred to as high-order white (in contrast to first-order white).
Raith, Raase & Reinhardt – February 2012
The first graphical presentation between retardation, crystal thickness and birefringence [Γ =
d*(n z '–n x ')] was published by Michel-Lévy (1888, Tableau des biréfringences in "Les
Minéraux des Roches", Paris). This interference colour chart which shows just over four
colour orders is used to this day as a useful standard tool for mineral determination at the
microscope. Improvements in printing technique over time ensured that modern colour charts
are a reasonably close reproduction of the interference colour spectrum observed in polarizedlight
microscopes (such as the colour charts of Zeiss and Leica). Nevertheless, some
weaknesses in colour reproduction are evident like the second-order green which is far too
prominent in most colour charts. The Michel-Lévy colour chart of this guide provides an
improved reproduction of the colour spectrum (Fig. 4-29). It has been calculated by Dr. Bjørn
Eske Sørensen (Department of Geology and Mineral Resources Engineering, NTNU-Trondheim,
Norway) using MATLAB. The spectrum has been calculated taking into account human
colour perception (CIE-calibrated curves of sensitivity for the primary colours red, green,
blue) and the RGB colour space of the computer. Conformity between calculated and
observed interference colours was further optimised using the gamma correction of the
intensity values.
When redesigning the Michel-Lévy colour chart for this guide we modified the presentation
such that the application principle for mineral determination is easy to grasp, compared with
the widely distributed standard charts of Zeiss or Leica (Figs. 4-29 and 4-32). Guided by our
experience with microscopy courses we also developed an alternative concept for the
interference colour chart. We believe this chart is easier to read where birefringence values or
crystal thickness need to be determined, compared with the classic colour chart. The difference
to the standard Michel-Lévy colour chart is that the colours representing retardation are
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