Module 2: Using the Petrographic Microscope

# 2.8 Interference Figures: Part 2

Elizabeth A. Johnson and Juhong Christie Liu

# Biaxial Interference Figures

The following two videos explain how to obtain biaxial interference figures, how to determine optic sign for a biaxial mineral, and how to estimate a 2V angle.

Figure 2.8.10. Earth Optics Videos. Video 6: Biaxial Minerals. CC-BY license. https://youtu.be/pA_fJnbPRtg

Figure 2.8.11. Earth Optics Videos. Video 7: Biaxial Flash Figure. CC-BY license. https://youtu.be/qCkljJ1Isjw

Biaxial interference figure diagrams. Figure 2.8.12.A. Different types of interference figures produced by biaxial minerals. For simplicity, isochromes are not shown. The lower part of each image shows the orientation of the indicatrix for the mineral. The middle part of the diagram shows the interference figure relative to the thin section and the upper image shows the view through the microscope ocular.M= melatope; Bxa = acute bisectrix; Bxo = obtuse bisectrix. Figure 2.8.12.B. Different types of interference figures produced by biaxial minerals. For simplicity, isochromes are not shown. The lower part of each image shows the orientation of the indicatrix for the mineral. The middle part of the diagram shows the interference figure relative to the thin section and the upper image shows the view through the microscope ocular.Bxa = acute bisectrix; Bxo = obtuse bisectrix.

Figure 2.8.13. An acute bisectrix optical interference figure obtained on albite.

Figure 2.8.14. An optical interference figure obtained on an augite crystal oriented parallel to the 001 crystallographic direction.

Figure 2.8.15.  An optical flash figure obtained on an augite crystal.

Guided Inquiry

# Interference Figures and Crystal Symmetry

The types of interference figures that a mineral produces are related to the crystal symmetry of that mineral.  The figure below summarizes the division of crystal symmetries into isotropic, uniaxial, and biaxial optical properties. Figure 2.8.16. The crystal systems labelled with optical properties (isotropic, uniaxial, or biaxial). If the symmetry of a crystal is known, it can be used to predict which type of interference figures the mineral will produce in thin section.

Guided Inquiry

# Synthesis: Is it Uniaxial or Biaxial?

Figure 2.8.17. An interference figure obtained on a calcite crystal.

Figure 2.8.18. An interference figure obtained on a nepheline crystal.

Guided Inquiry 