Chemistry Reference
In-Depth Information
applies and the interference colors of such minerals no longer belong to the
Michel-Lévy chart; such colors are referred to as anomalous colors: brown
gray, olive green, Prussian blue, lemon yellow, etc. There are not many min-
erals that produce such anomalous colors; recognizing these anomalous
colors is an important guide to their identification.
2.5.3 Position of the indicatrix - Angle
of extinction
According to the above model, extinction of the studied section occurs four
times per revolution of the stage, every time the n
of the section
coincide with the polarization planes of the nicols. Identify these directions
of extinction in relation to outstanding crystallographic elements (elonga-
tion, cleavage, twins) therefore allows us to locate the position the indicatrix
in relation to the lattice; it is all the more accurate as several sections have
been examined. Extinction can be parallel to one of these structural elements
(parallel extinction) or occurs at some angle to them (inclined or oblique
extinction). The angle between the direction of extinction (and, more spe-
cifically between
γ
and n
α
) and these structural elements is characteristic of
a given mineral and/or varies regularly depending on its chemical composi-
tion. Charts were established for accurate determinations and are commonly
used for the determination of plagioclase. The precise construction of these
structural elements and the indicatrix requires Fedorov universal stage.
γ
,
β
and
α
2.5.4 Sign of elongation
The directions of extinction allow the identification n
α
and n
γ
of a section.
The problem is how to recognize which one is n
. Let us consider a
remarkable direction of a given mineral: elongation, cleavage, twin, etc. It is
said that this direction has a
positive elongation if this direction is n
α
or n
γ
γ
of the
section
(or at least form an acute angle small enough with n
γ
),
the elonga-
tion is said to be negative, if it is n
.
To determine the sign of elongation, the stage is rotated at 45 degrees to
the direction of extinction in order to have a maximum intensity. An auxil-
iary plate, with a known birefringence and a known
α
direction is added (in
the slot of the microscope) between the objective and the analyzer in SE-NW
direction;
γ
direction of the plate has thus a SW-NE direction. The most use-
ful is a quartz plate whose birefringence is equal to
γ
0,018
(for a thickness of 30 micrometers) which corresponds to the boundary
between first and second order in the interference color chart (such quartz
plate is called in French “quartz teinte sensible”).
If n
δ
=
e(n
γ
−
n
α
)
=
) of the acces-
sory plate, that is to say that this is a positive elongation, retardations
induced by the mineral and the accessory plate add: interference colors of
γ
of the mineral has the same direction than the one (
γ
