guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Terminology & abbreviations
A note on terminology and some abbreviations used in this book
When using a polarized-light microscope, communicating directions in an unequivocal way is
important. The cross-hairs in the ocular, the directions of light polarization and the microscope
axis are the main reference directions. The four cardinal points (and intermediate directions
derived from those) are commonly used to express and distinguish directions, with no
geographical meaning, obviously. For a standard microscope set-up, "N-S" means parallel to
the "vertical" cross-hair or parallel to a line running from brow to chin between the two eyes
of the observer; "E-W" means parallel to the "horizontal" cross-hair or parallel to a line across
the centre of both eyes. Diagonal directions are thus referred to as NW-SE and NE-SW.
The Greek letters α, β, γ, ε, ω are used by some authors as subscripts to refractive indices (n).
In this text, we follow the terminology of Tröger et al. (1979) and others which is logical and
intuitive in the sense that the refractive indices n x , n y and n z correlate with the axes X, Y and Z
of the coordinate system in which the shape of the indicatrix of biaxial crystals is defined.
Furthermore, α, β, γ are crystallographic cell parameters, and hence any potential confusion
with optical parameters should be avoided. In the same way as in n x , n y and n z , the subscripts o
and e relate to refractive indices of O- and E-rays or O- and E-waves in uniaxial crystals.
As pointed out in the previous edition of the guidebook, many (though not all) authors of
optical mineralogy textbooks use the Greek letter delta for two different parameters that are
related to each other: δ stands for birefringence and ∆ for retardation (∆ = δ * d; d = thickness
of crystal plate). To add to the potential confusion, ∆ is the symbol for birefringence in the
widely used Tröger tables (Tröger et al., 1979). Although the use of δ and ∆ may be standard
symbols in the teaching of crystal optics in some institutions, we decided to abandon these for
the 2012 edition in order to make the shorthand terminology less confusing for those being
introduced to optical mineralogy. We will use the symbol ∆ strictly in its standard mathematical
sense (i.e., "difference"). Hence, birefringence can simply be expressed as ∆n (= n z - n x ,
for example). The symbol for retardation is the Greek letter Γ (hence, Γ = ∆n * d).
Raith, Raase, & Reinhardt – January – February 2012 2012
In science the common symbol for wavelength is λ, but in optical mineralogy λ is also used
for indicating the interference colour order (1λ = first order red, 2λ = second order red, etc.).
We have tried to circumvent reference to the latter, but the use of terms such as "λ-plate"
(meaning 1λ or 551 nm retardation corresponding to 1st order red) and "λ/4-plate" are somewhat
entrenched and thus hard to avoid.
IV