Chemistry Reference
In-Depth Information
weak, clearly lower than the one of quartz, like potassium feldspars
(1,514-1,526);
of the same order as quartz, like cordierite or oligoclase (plagioclase);
moderate to strong like muscovite (1,552-1,716), amphiboles (1,599-
1,730) or tourmaline (1,610-1,675);
strong like orthopyroxenes (1,650-1,788) or kyanite (1,712-1,734);
very strong like garnet (1,714-1834), epidotes (1,670-1,797) or zircon
(1,923-2,015).
Precise estimation of the relative relief of two minerals is made by the
Becke line
method. The microscope is focused on the contact of the two min-
erals, using a suitable magnification and possibly stopping down the aper-
ture of the diaphragm. If the objective is
moved away
from the thin section
(
increasing
distance) a bright fringe appears and moves towards the mineral
with
the higher index
(
with a higher relief). If the
objective is
moved closer
to the thin section (
decreasing
distance) the bright
fringe moves towards
the least refracting
mineral (
=
more refracting mineral
=
with a lower relief).
When a mineral has a high birefringence, that is to say two very differ-
ent indices
=
), the relief of the mineral will be very different
depending on its relative position to the plane of polarization of incident
light: it is the phenomenon of
change of relief
(in French: «pléochroïsme de
relief»). A classical example is calcite:
γ
and
α
(
ε
and
ω
γ
is much lower than quartz (1,486)
and
0,172); by rotating the stage, the calcite
appears either with a negative relief in comparison with quartz, or in posi-
tive relief.
α
much higher (1,658) (
Δ
=
2.4.3 Color - Pleochroism
When a beam of white light passes through a given mineral, part of the
incident light is absorbed. The absorption coefficient depends on the wave-
length. The mineral appears with the color complementary to that which
has been absorbed. Some minerals are
opaque
, others entirely transparent
for all the wavelengths (so they appear
colorless
), others
colored
(with a
distinctive color).
In anisotropic media, the absorption coefficient depends on the direc-
tion of the light transmitted: as the incident wave is plane polarized, the
mineral appears colorful, more or less colored or even shows very differ-
ent colors depending on the relative position of section and of the plane of
polarization - this phenomenon is
pleochroism
. For instance, the color of
biotite varies from reddish brown to pale brown, that of piemontite from
carmine to orange.
The most intense color is observed either in the NS direction or in the
EW direction of the cross hairs of the reticle, depending on the mineral
and position of the plane of polarization of the polarizer. Some minerals
