Observations with the petrographic microscope - Mineralogy for Petrologists - page 43

γ γ

accessory
plate

accessory
plate

Blue Red

Blue

Red

Red

Blue

Blue

Red

Blue

Red

Red Blue

α is the bisectrix of the optic axes

2V < 45° => Ne gative Biaxial B−

γ is the bisectrix of the optic axes
2V < 45° => Positive Biaxial B+

Figure 2.17 Optic sign.

r > b

r

b

b

r

r = hyperbola in red light (dotted line)
b = hyperbola in blue light (continuous line)

Figure 2.18 Dispersion.

ference color chart) and NW and SE quadrants red-orange colors (lower
in the interference color chart);

￿ it is the inverse if is the bisector of the acute angle of the optical axes:

mineral is biaxial negative.

α

2.6.5 Dispersion
In some biaxial minerals, the angle of optical axes varies with wavelength.
The figure made of convergent light on a section perpendicular to the
bisector of the optical axes is a rectangular hyperbola passing through the