Chemistry Reference
In-Depth Information
N
γ
accessory plate
(for instance here a quartz
with interference color
at the boundary
between first and
second order)
accessory plate
γ
n
γ
n
α
n
α
bright colors of the
2nd−3rd orders
first order
gray, pale yellow
n
γ
n
α
of the mineral and
γ
of the auxilliary
plate coincide: retardation are subtracted:
interference colors are lower in the
Michel-Lévy chart
n
γ
coincides with the elongation (for example a cleavage): positive elongation
n
γ
of the mineral and
γ
of the auxilliary
plate coincide: retardation are added:
interference colors are higher in
Michel-Lévy chart
Figure 2.9
Sign of elongation.
the mineral are added to the ones of the accessory plate; the result is that
the interferences colors of the whole are higher in the Michel-Lévy chart; in
comparison with the purple/indigo interference color of the accessory plate,
we get 2nd-3rd order bright colors.
Countercheck: we rotate the stage 90°: now n
α
of the mineral coincides
with
of the accessory plate. Retardations are subtracted and interference
color is lower in the Michel-Lévy chart: the result is a 1st order orange yel-
low or yellowish gray.
The opposite is observed in the case of a negative elongation: bright
colors when the section has a NW-SE direction, pale first order colors in
the direction SW-NE.
In the case of minerals with interference colors of second to third order,
using a mica plate (in French “mica quart d'onde”
γ
quarter-wave) is used;
its thickness corresponds to a birefringence of a quarter of an order, so that
it produces a medium gray interference color. The approach is similar to
that used with an accessory plate of quartz.
=
2.5.5 Twinning
A twin is an intergrowth of two or several crystals of the same substance,
joined by a symmetry law:
in
contact twins
the symmetry in relation to a plane, the
twinning plane
;
the contact is then a plane: for instance albite twin in plagioclases;
in
penetration twins
the symmetry is in relation to an axis, the
twinning
axis
, the two crystals are joined on an
irregular contact
: for instance
Carlsbad twin in the feldspars.
