This document discusses the different forms that uniaxial interference figures can take based on the orientation of the optic axis in a mineral sample. There are three main types: 1) Optic axis figures where the optic axis is vertical and the cross pattern is centered. 2) Off-centered figures where the cross is not centered and moves as the sample is rotated. 3) Optic normal figures where the optic axis is horizontal, producing a broad, fuzzy interference pattern across most of the view. Key elements like isogyres, melatope, and isochromes are also defined.

1. Forms of Uniaxial
Interference Figures
Prathamesh S. Pawar
M.Sc. (I) Geology,
Dept of Geology,
S. P. Pune University

2. Nomenclature
• Isogyres: black bars which form the cross
• Melatope: intersection of the two isogyres.
It is the point from
where the optic
axis emerges.
• Colour bands are
called as
isochromes.
Interference
colours increase in
order outward from
the melatope, near
melatope colours
are low first order.

3. Forms or Types of Uniaxial Interference
Figures
• 3 types – Based on the orientation of optic
axis, which is analogous the orientation of the
mineral section
1. Optic Axis or Centred Figure
2. Off-Centred Figure
3. Optic Normal or Flash Figure

4. 1. Optic Axis Figure
• aka Centered Interference Figure
• The isogyres form a perfect cross
• Melatope is perfectly centered
• Optic axis is vertical
• Interference figure for the mineral does not
move as the stage is rotated

6. Some examples of centered uniaxial
interference figures
Calcite Quartz

7. 2. Off-cantered Figure
• Cross is not centered
• Melatope may or may not lie outside the field
of view
• Optic axis is not vertical
• On rotation the melatope swings in a circle
around the centre

8. If the Melatope is well out outside the field of
view the isogyres sweep across the field of
view in sequence as the stage is rotated - with
the isogyres always remaining parallel to the
crosshairs.

9. Some examples of of-centered uniaxial
interference figures

10. 3. Optic Normal Figure
• aka Flash figure
• optic axis
horizontal
• The interference
figure produced is
very broad, fuzzy
and occupies most
the field of view

11. Thank You…