Chemistry Reference
In-Depth Information
Variations with (n
γ
−
n
α
)
This value is characteristic of the section of the studied mineral. This value
varies for different sections of the same mineral, as n
γ
−
n
α
=
(
γ
−
α
) sin
θ
sin
(see paragraph 2.1) from 0 for the circular section(s) to a maximum
value:
θ′
Δ
=
γ
−
α
The observed interference colors vary from black (no light) for circular
sections up to a maximum value corresponding to the birefringence of the
mineral: this value is characteristic of the mineral. The Michel-Lévy chart
shows these interference colors as a function of the birefringence of the min-
eral,
and thickness of the thin section, e. Remember that in most cases this
thickness is 30 micrometrers.
Δ
2.4 OBSERVATIONS IN PARALLEL POLARIZED
LIGHT (PPL)
2.4.1 Forms, fractures, cleavages
A mineral may be
euhedral
(idiomorphic) with its own crystalline forms, or
anhedral
(without any crystalline forms).
A
cleavage
is a separation plane, more or less perfect, regular, repeated,
that corresponds to a zone of weakness in the lattice of the mineral. Cleavage
may be more or less good: perfect thin lamellar cleavage in muscovite, good
in amphibole or in pyroxenes, poor in nepheline. The same mineral can have
several cleavages, which are not necessarily the same quality: some perfect,
others not. The cleavages follow the same laws of symmetry as the crystal.
A
fracture
is a break that is not planar in general, and overall is not
repeated regularly, or at least, is repeated a few times. The fractures of some
minerals, such as tourmaline, may be characteristic.
2.4.2 Index/refringence
The index of a mineral can be appreciated by the effect of relief of this min-
eral compared with the surrounding minerals. One can estimate this index
by comparison to that of Canada balsam (1.535-1.542), or araldite (1.545),
commonly used to glue the cover slip on the preparation, or in comparison
with a common mineral like quartz (1.544-1.553).
Practically various indexes may be distinguished:
very weak like fluorite (1,433-1,435) which shows a strong negative
relief;
