Chemistry Reference
In-Depth Information
are the most intense colors in the same direction as the most intense shades
observed in biotite: it is called
direct pleochroism
. Others, such as tourma-
line, have less intense shades in the directions where those of biotite are the
most intense (and vice versa): this is called
reverse pleochroism
.
2.4.4 Inclusions
Some inclusions are characteristic of the host mineral. For instance the inclu-
sions of zircon in the cordierite are surrounded by a characteristic yellow
pleochroic halo.
Others allow us to reconstitute the history of the mineral. For instance,
euhedral oriented inclusions of plagioclase in phenocrysts of orthoclase wit-
ness the magmatic evolution.
Fluid inclusions are particularly interesting in petrology. Their study
requires special skill and equipment.
2.4.5 Alterations
Alterations allow us to reconstruct the late history of a mineral, and thus
the rock.
Some alterations are characteristic of some minerals and are thus a cri-
terion of determination: for example cordierite is altered to yellowish iso-
tropic chlorites; red clouding is characteristic of orthoclase.
2.5 OBSERVATIONS IN CROSS POLARIZED
LIGHT (CPL)
2.5.1 Interference colors - Birefringence
The thickness of the thin sections being fixed at 30
, the interference colors
of a mineral enable us to estimate its birefringence. The birefringence of any
section ranges from 0 to
μ
Δ
=
γ
−
α/
(
ε − ω
). Only the colors corresponding to
the maximum birefringence
Δ
=
γ
−
α
are significant. Sections perpendicular
to an optical axis (n
0) are always extinct. The sections of low bire-
fringence, which are almost perpendicular to an optical axis (n
γ
−
n
α
=
γ
−
n
α,
very
low), can be confusing.
2.5.2 Anomalous interference colors
The position of the indicatrix of some minerals (chlorite, epidote, chloritoid,
melilite), depends on the wavelength (in French such minerals are called
“minéraux dispersifs”, there is no correspondent expression in English). The
above calculation integrating the light intensity as a function of
λ
no longer
