Observations with the petrographic microscope - Mineralogy for Petrologists - page 22

d'Histoire Naturelle, College de France): theory of crystalline optics and
its application to petrographic microscope was firmly established by the
work of Fouque and Mallard (Cristallographie géométrique et physique
( Geometrical and Physical Crystallography ), 1884-1885), mineral synthe-
ses by Daubree at the Ecole des Mines, works: of Des Cloizeaux (Méthode
de détermination des plagioclases ( Method for determining Plagioclases ),
1875), Fouqué (Fouqué and Michel-Lévy: Minéralogie micrographique
( Micrographic Mineralogy ), 1879), Michel-Lévy (Etude sur la détermina-
tion des feldspaths dans les plaques minces au point de vue de la classifica-
tion des roches ( Study on the determination of feldspars in thin sections
from the point of view of the classification of rocks ), 1894-1904) and Lac-
roix (Michel-Lévy and Lacroix: les Minéraux des roches ( The Minerals of
the Rocks ), 1888). The main methods of microscopy applied to the study
of minerals and rocks were acquired in 1900.

2.1 INDICATRIX (REFRACTIVE INDEX
ELLIPSOID)

Light is an electromagnetic vibration. In an optically isotropic medium, the
electromagnetic wave propagates in a straight line and the electromagnetic
vibration is perpendicular to the direction of propagation. The velocity of
propagation of the light in vacuum is c. The velocity of propagation of the
light in a given medium is c/n, where n is the index of this medium. In an
optically isotropic medium, light propagates without distortion, with the
same velocity in all directions.
In an isotropic medium, the electromagnetic vibration may be the
same in all directions, it is called non-polarized light or circular polarized
light . In the case of plane polarization , the electric vector (and the mag-
netic vector that is perpendicular to it) vibrates in a single plane. In the
case of elliptical polarization , the electric and magnetic vector rotate
around their axis and their amplitude varies so that it describes an
ellipse. Plane polarized light may be obtained by different processes: an
appropriately cut prism of Iceland spar (transparent crystallized calcite)
called Nicol prism or Nicol (1829) has been used; and nowadays we use
polaroids made of organic crystals oriented included in a transparent
film (1929-1938).
In an anisotropic medium, the light vibration does not propagate with-
out distortion. But Fresnel theorems show that for any mineral section,
there are two perpendicular directions in which light propagates without
distortion at speeds c/n .

If we change this section in all directions, the locus of the vectors n

γ and c/n α

γ

and n , is an ellipsoid, called the refractive index ellipsoid or indicatrix

(Figure 2.1).

α