Chemistry Reference
In-Depth Information
Twins may by simple or repeated. Among the repeated twins it is
distinguished:
Polysynthetic
twins: the joined individual crystals form parallel lamel-
lae: twins in high temperature cordierite; albite and pericline twins in
plagioclases
Cyclic twins: individual crystals form a more or less circular associa-
tion: twins of leucite, analcime, cordierite, etc.
Under the microscope, twins are shown by differences of interference
colors of the associated crystals.
Syneusis
(textures peculiar to igneous, and more commonly to volcanic,
rocks where several crystals are associated in any manner in the core of
the structure, the whole being surrounded by a common envelope),
Growth
forms
and
deformation of minerals
have more to do with the study of the
texture of the rocks than with the determination of minerals.
2.6 OBSERVATIONS IN CONVERGENT
POLARIZED LIGHT
Previous observations were made with a parallel light beam. The addition
of the condenser between the light source and (lower) polarizer and the thin
section, transforms the light beam into a (slightly) conical beam. The obser-
vation is done with the upper polarizer (analyzer) in place.
The various narrow light beams that form this cone crosscut the studied
section in different directions: they pass through
variable thicknesses
of the
section and meet the index ellipsoid with a
variable incidence
. At the exit of
the section, each light beam therefore presents different interference colors
and intensities. The recomposition of the different light beams produces a
complex interference pattern composed of:
isochrome lines
where the vibrations are in phase, these curves are show
interferences colors in accordance with the succession Michel-Lévy
chart. The isochromes can be clearly seen only if the mineral has a high
birefringence (calcite for example) or if the section is (too) thick;
isogyres
(in French, “lignes neutres”
neutral lines) where the ampli-
tude of vibration is null and therefore appear black. The points of emer-
gence of the optic axes (or axis) belong to the isogyres; it is sometimes
called melatope.
=
Whilst in the preceding observations we looked at the image given by
the ocular, the obtained image is in the focal image plane of the objective.
Therefore, to observe this image, we have either to remove the ocular or
