guide to thin section microscopy - Mineralogical Society of America

Guide to Thin Section Microscopy
Conoscopy
4.2.5 Conoscopic methods
4.2.5.1 Some basic principles
In "normal" orthoscopic view light enters the thin section orthogonally as a set of parallel,
E-W vibrating light waves. The objective creates an enlarged, real, inverted image of the thin
section, which is then further enlarged and viewed through the ocular. The light rays entering
the thin section travel through the mineral grains in defined crystallographic directions that
depend on the respective orientation of the crystals. In order to record the optical behaviour
of an anisotropic mineral species, crystal sections of different orientations must be studied, as
outlined in previous chapters (e.g., crystal sections with maximum interference colour are
needed to determine birefringence).
Optic axial angle and optic sign cannot be determined directly in the orthoscopic mode of
operation, except in specific circumstances where the crystal symmetry is known to be high
(hexagonal, trigonal, tetragonal) and where the crystal shape allows to identify the c-axis
direction (cf. Ch. 4.2.4).
As opposed to orthoscopy, the conoscopic view involves a convergent set of light rays, which
means that a wide-angle cone of differently inclined light rays transmits the thin section (Fig.
4-47). To achieve this, the illumination aperture is enlarged to a maximum by putting the
auxiliary condenser lens into the light path and opening the substage iris diaphragm.
Raith, Raase & Reinhardt – February 2012
Dependent on their propagation direction, the rays of this light cone will form an image in the
focal plane above the objective, whereby the maximum opening angle of the cone of rays
forming that image depends on the aperture of the objective used (Fig. 4-48). Since two
orthogonally vibrating waves propagate generally in all directions within optically anisotropic
minerals, these wave couplets are brought into interference in the analyzer, generating an
interference figure. This interference figure can be viewed, either as an enlarged image by
putting an auxiliary lens (Amici-Bertrand lens) into the light path, or with the naked eye by
removing an ocular and looking down the ocular tube. Light waves that travel in the direction
of the microscope axis, and are thus oriented orthogonal to the thin section plane, form the
centre of the interference figure. The larger the angle of the ray propagation direction to the
microscope axis, the larger the distance of the image point from the centre of the interference
figure. Thus, the conoscopic interference figure allows to study the propagation behaviour of
light in an anisotropic crystal quasi-simultaneously for a multitude of crystallographic
directions within that crystal.
From the geometry of the interference figure and its modification through the use of
compensators, the number of optic axes (uniaxial vs. biaxial), the optic axial angle (2V) and
the optic sign (positive vs. negative) can be determined.
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