guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
Skiodrome sphere
For the optically uniaxial mineral species, a net of longitudinal and latitudinal circles, which
has rotational symmetry, forms the surface of the sphere (Fig. 4-49). The meridians (longitudes)
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.
For the optically biaxial mineral species, the spherical surface consists of an arrangement of
two sets of cross-cutting ellipses with orthorhombic symmetry (Fig. 4-50). The optic axes A 1
and A 2 emerge at the common foci of the ellipses. The symmetry planes of the net correspond
to the principal sections ZX, ZY and YX.
Skiodrome net (projected)
For practical reasons, the part of the surface of the skiodrome sphere that is captured in the
conoscopic cone of light rays as it emerges from a crystal sections is projected onto the thin
section plane (Fig. 4-49,50). The skiodrome nets and the corresponding interference figures
for diagnostically important orientations of optically uniaxial and biaxial minerals are
presented in Figs. 4-51 to 4-54.
Interference figure
In the interference figure, those domains are in extinction in which the vibration directions of
the waves are parallel to the polarizer and analyzer directions. [Analogy to the orthoscopic
extinction position: light entering the crystal in such directions will vibrate exclusively E-W
and hence will be blocked by the analyzer; N-S vibrating waves are not generated!]. The
domains of extinction are called "isogyres". Their geometry is dependent on the optical
symmetry and the orientation of the crystal, and it may change as the microscope stage is
turned (Figs. 4-51 to 4-54).
Raith, Raase & Reinhardt – February 2012
The domains in which the vibration directions of the waves deviate from the polarizer and
analyzer directions appear in different brightness and different interference colours. [Analogy
to the orthoscopic diagonal position: the light entering the birefringent crystal plate in such
directions is split into two sets of orthogonally vibrating waves, which interfere afterwards in
the analyzer]. Curved lines of equal colour (or equal retardation Γ) are called "isochromes".
Their geometry depends on the optical symmetry and the orientation of the crystal (Fig. 4-
51,53). The range of the colour spectrum, i.e. the number of isochromes, is determined by the
birefringence values, the thickness of the crystal plate [Γ = d * (n z ' – n x ')] and the aperture of
the objective (Figs. 4-52 to 4-54).
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