guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Extinction
Sign of elongation l
Crystals with columnar to acicular or platy to flaky habits show elongate sections if cut
parallel to their long morphological axis or perpendicular to a dominantly developed basal
plane. If such elongate sections are rotated into a diagonal position (i.e., long axis l parallel to
NE-SW) they show under crossed polarizers and compensator-in (first-order red plate or λ/4
plate) either addition (Γ Comp + Γ Min = Γ total ) or subtraction (Γ Comp - Γ Min = Γ total ), depending on
their optic sign (Fig. 4-45).
Addition is observed if the wave with the larger refractive index (n z ') vibrates parallel or at a
small angle to the long dimension of the crystal section: l (+) = positive sign of elongation or
"length-slow". Examples: acicular-columnar crystals– sillimanite, gedrite, anthophyllite; platy
crystals– gehlenite, micas, antigorite, pennine.
Subtraction is observed if the wave with the smaller refractive index (n x ') vibrates parallel or
at a small angle to the long dimension of the crystal section: l (–) = negative sign of
elongation or "length-fast". Examples: acicular-columnar crystals– tourmaline, apatite; platy
crystals– åkermanite, brucite, clinochlore.
For optically uniaxial mineral species with acicular-columnar habit, the sign of elongation
corresponds to the optic sign. For platy uniaxial minerals, the two signs are opposite.
Elongate sections in which n y is parallel (or at a small angle) to the morphological long axis
of the crystal or perpendicular to the planar dimension of platy crystals show a variable sign
of elongation, l(+) or l(–), depending on the exact orientation of the crystal in thin section.
Examples of such elongate crystals– wollastonite, epidote.
Raith, Raase & Reinhardt – February 2012
Figure 4-45 A. Platy muscovite showing positive sign of elongation. In muscovite and firstorder
red plate, fast (n x ) and slow (n y ↔n z ) waves vibrate in identical orientation. Therefore,
the retardation adds up and increased interference colours result (Γ res = Γ Red1 + Γ Ms = 1231 nm;
second-order blue-green).
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