| Description |
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.
- Introduction
- Crystal systems and symmetry; review of Miller indices
- Properties of light and polarized light; dispersion
- Refraction and Snell's law; velocity vs. refraction; relief
- Isotropic and anisotropic substances, and the relation of crystal symmetry to optical properties; calcite experiment
- Anisotropic optics: Extinction, retardation, phase interference phenomena, birefringence, extinction
- The optical indicatrix; isotropic and anisotropic (uniaxial/biaxial)
- The 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 dispersion
- Refractometry: 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.
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