Chemistry Reference
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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.
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