Chemistry Reference
In-Depth Information
direction
of propagation
γ
random
section
n
γ
α
β
n
α
Figure 2.1
Indicatrix.
for biax-
ial minerals (Notation by Deer, Howie and Zussman, 1966-1992). Those
indexes may also be noted respectively N
This ellipsoid is defined by its three principal axes
α
,
β
and
γ
, or X, Y, Z in Tröger
(1971)) or Np, Nm and Ng (French notation). The notation of the indexes
for an uniaxial mineral are
α
, N
β
and N
γ
(perpen-
dicular to it) (note that this anglo-saxon notation is rather inconsistent).
Birefringence
is the quantity
ε
(principal axis of symmetry) and
ω
Δ
=
γ
−
α
for biaxial minerals or |
ε
−
ω
|
(absolute value) for uniaxial minerals, (or, in any case, Ng
−
Np according
to the French notation).
The indicatrix has the symmetry of the medium and its axes of symme-
try coincide with those of the mineral:
In a cubic mineral, it is a sphere: a cubic crystal is optically isotropic.
In a mineral having an axis of symmetry of order greater than 2 (hexagonal,
trigonal, tetragonal systems), the indicatrix is an ellipsoid of revolution.
In the orthorhombic, monoclinic and triclinic systems, the indicatrix is
a triaxial ellipsoid.