Chemistry Reference
In-Depth Information
add a device, the Bertrand lens, used to observe the image through the
ocular.
The pattern of the interference figure depends on the orientation of the
section. It is only for specific sections that these figures are easily interpret-
able: sections perpendicular to an optic axis and sections perpendicular to
the bisector of an optic axis. Do not to try to interpret a figure too far from
these classic cases!
2.6.1
Obtain an interference figure
1
choose the section studied among the different sections of the mineral
present in the thin section - this is the most difficult -
A section perpendicular to an optical axis remains always extinct in
cross polarized light;
If you look at a section perpendicular to the bisector of the optic
axes, you must have an idea about the nature of the mineral and its
structure: for example, in sodic plagioclase these sections are almost
perpendicular to the two cleavages.
2
Use the higher magnification so that the studied mineral occupies
the entire field and perfectly focus, the objective must be perfectly
centered.
3
Place the analyser, if it not already done.
4
Flip the condenser and fit it so that its lens is almost touching the
section.
5
The observations are made either by removing the ocular (and possibly
using an eyepiece) or by placing the Bertrand lens.
2.6.2 Uniaxial mineral: section perpendicular
to the optic axis
The isogyres are two straight lines parallel to the planes of polarization of
the Nicols (and so to the cross hairs of the reticle) which form a black cross,
perfectly centered if the section is effectively perpendicular to the optical
axis. This black cross does not move when you rotate the stage.
The isochromes are circles centered on the axis of the microscope.
If the section is not quite perpendicular to the optic axis, the black
cross is not centered at the center of the field (crossing of the cross hairs)
but its center is at the trace of the optic axis in the field (melotope). When
you turn the plate, the trace of the optic axis describes a circle and the black
cross moves parallel to itself. Strictly speaking, isochromes are no longer
circles.
Such a figure may also be interpreted if the center of the cross is out of
the field. Anyway, you have to be very cautious in the interpretation.
