Identification
of crystalline materials with the polarized light microscope is of
importance for those working in forensics, soil science,
pharmaceuticals, environmental analyses, analysis of pigments, and earth
sciences. This class introduces the student to a range of methods used
for the identification of crystalline materials. A number of techniques
are employed or demonstrated, including immersion methods, crystal
rotation methods and the use of rotating compensators. Optical
properties of both isotropic and anisotropic comminuted mineral samples
are used as examples throughout the course.
Course Outline:
IntroductionCrystal systems and symmetry; review of Miller indicesProperties of light and polarized light; dispersionRefraction and Snell's law; velocity vs refraction; reliefIsotropic and anisotropic substances, and the relation of crystal symmetry to optical properties; calcite experimentAnisotropic optics: Extinction, retardation, phase interference phenomena, birefringence, extinctionThe petrographic microscope; components, and adjustment for orthoscopic and conoscopic observation.Properties
observable with the petrographic microscope: Twinning, pleochroism,
cleavage, birefringence, sign of elongation, extinction angles; relation
to crystal symmetry, anomalous interference colors, interference
figures.Formation
of interference figures: Optic axis, off-center optic axis, Bxa, Bxo,
optic normal (flash), random figures; use of angle of entry and exit of
hyperbola (AEEH) to diagnose centered figures. Recognition of
interference figures and application to interpretation of crystal
orientation in uniaxial and biaxial minerals: Optic sign; determination
using both a waveplate and a quartz wedge, optic angle (2V) and relation
to indices of refraction in biaxial minerals, optic axis dispersion;
crossed axial plane dispersionRefractometry:
Identification of crystalline materials using calibrated index liquids
and the colored Becke line; cause of Becke line colors.Properties of the more important rock-forming and accessory minerals in thin section: Silica-group minerals; polymorphs of SiO2,
feldspar minerals; K-spar and plagioclase groups, amphibole- and
pyroxene-group minerals, phyllosilicates, carbonates, accessory
minerals.Advanced
methods of crystal identification: Measurement of retardation by use of
rotary compensators, spindle stage, universal stage.Ore
microscopy: Demonstration of basic optical properties (reflectance,
bireflectance, anisotropism, reflection pleochroism, Kalb line, etc.)
that can be measured in reflected light, and their application to
identifying opaque minerals or characterizing metallurgical samples;
comparison of the properties of reflected and transmitted light.
