3.2.5 beta (β)—symbol representing the intermediate prin-
cipal refractive index of a biaxial crystal.
3.2.6 biaxial, adj—an anisotropic crystal in the
orthorhombic, monoclinic, or triclinic system with three prin-
cipal refractive index directions (α,β,γ) and two optic axes
that are isotropic.
3.2.7 birefringence, n—the numerical difference between
the maximum and minimum refractive indices of anisotropic
substances.
3.2.8 crystal system, n—crystals are classified according to
their crystallographic axes length and the angles between them.
There are six crystal systems: cubic, tetragonal, hexagonal,
orthorhombic, monoclinic, and triclinic. All crystal systems are
anisotropic except for the cubic system, which is isotropic.
3.2.9 epsilon (ε) —any vibration direction in the plane of the
c axis for uniaxial crystals.
3.2.10 euhedral, adj—well-formed crystals bounded by
plane faces.
3.2.11 extinction, n—condition when an anisotropic sub-
stance appears dark when observed between crossed polarizers
and occurs when the vibration directions of the specimen are
parallel to the vibration directions of the polarizer and analyzer.
3.2.12 extinction, oblique, n—a type of extinction in which
the vibration directions are inclined at an angle relative to a
crystal face.
3.2.13 extinction, parallel, n—a type of extinction in which
the vibration directions are parallel to the crystal faces.
3.2.14 extinction, symmetrical, n—a type of extinction in
which the vibration directions bisect a prominent interfacial
angle of the crystal.
3.2.15 fusion methods, n—A process in which compounds
are heated on a microscope slide and observed via PLM during
heating, melting, and, upon cooling, recrystallization of the
melt.
3.2.15.1 Discussion—Fusion methods frequently employ a
temperature controlled hot stage capable of at least 300 °C
placed on the stage of a polarizing light microscope.
3.2.16 gamma (γ)—symbol representing the highest of the
three principal refractive indices of a biaxial crystal.
3.2.17 interference figure, n—pattern observed during cono-
scopic observation of an anisotropic material which consists of
a combination of extinction positions and interference colors
corresponding to the full cone of directions by which the
sample is illuminated.
3.2.17.1 Discussion—Conoscopic observations are typically
made by inserting a Bertrand lens into the body tube or by
removing an eyepiece and viewing down the body tube.
3.2.18 omega (ω)—any vibration direction in the plane of
the a axis for uniaxial crystals.
3.2.19 optic axial angle (2V), n—the acute angle between
two optic axes of a biaxial crystal.
3.2.20 optic axis, n—a direction of isotropic light propaga-
tion in an anisotropic crystal. Uniaxial crystals have one optic
axis; biaxial crystals have two optic axes.
3.2.21 optic sign, n—determined by the relationship of the
refractive indices of a material. For uniaxial crystals, if ε>ω,
the crystal is positive (+); if ω>ε, the crystal is negative (-).
For biaxial crystals, if γ-β>β-α, the crystal is positive (+); if
γ-β<β-α, the crystal is negative (-).
3.2.22 polymorphism, n—crystallization into two or more
chemically identical but crystallographically distinct forms.
3.2.23 relief, n—contrast between a particle or crystal and
its media due to the difference between their refractive indices.
The greater the numerical difference in refractive indices, the
greater the relief.
3.2.24 subhedral, adj—crystals with imperfectly developed
faces.
3.2.25 uniaxial, adj—an anisotropic crystal in the tetragonal
or hexagonal system having one optic axis (isotropic direction)
and either two (tetragonal) or three (hexagonal) directions
which are alike and perpendicular to the direction of the optic
axis.
4. Summary of Practice
4.1 Solid chemical components in samples are analyzed
using a polarized light microscope (PLM). The optical crystal-
lographic properties such as refractive index, birefringence,
and morphology are determined. Determination of these
properties, which are different for various explosive compo-
nents and other chemical compounds, can aid in the identifi-
cation of explosives and explosive residues submitted to the
forensic laboratory.
4.2 Optical crystallographic properties of an unknown com-
ponent can be determined by either mounting the component in
a refractive index liquid, recrystallizing the component from a
small drop of solvent, or recrystallizing the component from its
melt, that is, recrystallization occurring when a melted com-
pound is cooled. Recrystallized components can be isolated for
further analysis using other analytical techniques such as
Fourier transform infrared spectroscopy (FTIR), Raman
spectroscopy, or scanning electron microscopy-energy disper-
sive x-ray spectroscopy (SEM-EDS).
4.3 Water-soluble components, such as inorganic salts used
as oxidizers, are dissolved in a drop of water and tested using
reagents to form characteristic microcrystals. Microcrystal
tests can aid in the determination of anions and cations present
in oxidizers and other components of explosives.
4.4 The optical crystallographic properties of an inorganic
compound and microcrystal testing for its ions are independent
techniques that provide complementary information. A combi-
nation of tests that includes the optical crystallographic char-
acterization of a compound and microchemical tests to confirm
the anion and cation are used to identify oxidizers and other
salts present in explosives.
4.5 A combination of tests that include optical crystallo-
graphic characterization or microchemical tests in conjunction
with other independent techniques are used to identify chemi-
cal components present in explosives. Refer to Practices E3253
and E3329 for the requirements for the identification of
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