Glass Fiber and Phytolith
Transmitted Oblique Off Crossed Polarized Light Illumination

Glass fiber and silaceous phytoliths are both isotropic, dark between crossed polarizing filters.
A glass fiber and a phytolith are both present in the tapelift from a school in Eastern Washington State.
Oblique illumination provides a very quick way to distinguish the difference between most glass fiber
and silica phytoliths. The glass fiber genrally has a refractive index greater than the tape adhesive.
The tape adhesive (acrylic) has a refractive index of about 1.49. Most common glass fiber has a refractive
index of 1.51 or higher, often much higher, fused quartz fiber is the only common exception with a
refractive index of 1.48. Oblique illumination results in a shadow effect and the shadow falls toward
the source of the light or away from the source of the light depending on whether the refractive index
of the particle is higher or lower than the mounting medium. Phytoliths, with a refractive index of
1.48, have a refractive index below the acrylic adhesive. In this photograph the phytolith in the upper
part of the photograph is shaded on the left side. The glass fiber at the lower left is shaded on the
the right side. This shows that the glass fiber has a higher refractive index than the phytolith. The
intensity of the shadow can be used in the same way as "relief" with normal brightfield illumination
but with the advantage that no change in focus in necessary to characterize the Becke Line. This is
a great aid in quickly characterizing particles as one scans the sample. On closer inspection of the
phytolith it can be seen to have a rough or irregular surface. That is not consistent with formation
as a result of surface tension.