guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
4.2.5.3 Optically uniaxial minerals
For the optically uniaxial mineral species a net of meridional and latitudinal circles, which
has rotational symmetry, forms the spherical surface of the skiodrome (Fig. 4-49). The
meridians represent the vibration directions of the E-waves and the circles of latitude the O-
waves. The optic axis A is the rotation axis of the net.
Crystal sections perpendicular to the optic axis A are recognised in orthoscopic view by
their isotropic behaviour. In conoscopic view, these sections show a characteristic interference
figure which is a black cross. The isogyres become broader towards the margin of the
field of view, and the four sectors are filled by concentric coloured circles. The optic axis
forms the centre (melatope) of the cross (Figs. 4-49,51,52). If the microscope stage is turned,
the interference figure does not change.
Interpretation of the interference figure
The vibration directions of the O- and E-waves are orthogonal to each other and are arranged
concentrically around the optic axis. In the N-S and E-W directions of the interference figure,
the vibration directions coincide with the polarizer and analyzer directions. In these directions
only E-W vibrating waves pass the crystal plate (O-waves along the N-S isogyre; E-waves
along the E-W isogyre), but these will be blocked by the analyzer. Due to the rotational
symmetry of the indicatrix (and the skiodrome net) and the particular orientation of the
crystal plate (viewing direction parallel to the optic axis), the interference figure does not
move or change if the stage is rotated.
Raith, Raase & Reinhardt – February 2012
The increase of the interference colours towards the edge of the field of view is caused by the
increase of the retardation Γ of the O-waves and E-waves from the centre [∆n = 0 in the
direction of the optic axis] to the periphery, whereby the potential range of visible colours is
limited by the mineral-specific maximum birefringence perpendicular to the optic axis. [∆n =
|n e – n o |]. The circular arrangement of isochromes is a result of the rotational symmetry of the
indicatrix and the specific crystal orientation perpendicular to the optic axis. The range of the
colour spectrum (i.e, the number of isochromes) depends on birefringence and crystal plate
thickness [Γ = d * (n e ' – n o )] as well as on the numerical aperture of the objective. For
example, in a thin section of 25μm thickness, the interference figure of highly birefringent
calcite (∆n = 0.172) comprises about six orders of interference colour, while quartz (∆n =
0.009) shows first-order colours only (Fig. 4-52).
In crystal sections oblique to the optic axis A, the isogyre cross shifts towards the periphery
of the field of view and moves in a circular fashion if the microscope stage is rotated. The
distance of the melatope to the centre of the field of view reflects the tilt angle between the
optic axis and the microscope axis. The E-W and N-S portions of the isogyre cross move
through the field of view without rotation (Fig. 4-51 B,C).
Crystal sections parallel to the optic axis show maximum birefringence in orthoscopic view.
The conoscopic interference figure shows a broad black cross which, at minimal rotation of
the stage, quickly opens up ("flash figure“, Fig. 4-51D). Due to their similarity with
interference figures of optically biaxial minerals, interference figures of such orientations
have no practical significance.
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