PHYTOLITHS : RECOGNITION
The first step when a phytolith slide is put under the microscope is to recognise which ones of the observed particles are phytoliths.
Phytoliths are made of opal silica, which is amorphous and has a variable water content (SiO2·nH2O). Therefore anything under the microscope with the refractive index (RI) of opal silica (around 1.42) can be in principle a phytolith. If phytoliths are mounted in a higher RI they will have a distinctive greater relief than the mounting media. However, the observation of a series of characteristics can further help the researcher in correctly identifying a phytolith from any other amorphous silica particle (at least most of the time).
- Refractive index around 1.42
- Polarized light help in distinguishing phytoliths (opal silica) from quartz (crystalline silica). When opal is observed in cross-polarized light it is completely black and disappear from view.
- Morphology - remember that phytoliths are often exact copies of the cells in which the opal silica has been deposited and therefore they have inherited the shape of a plant cells. Therefore some knowledge of plant cell morphology can help in identifying phytoliths.
- Size - their size varies between 5 and 500 μm.
- The 3D shape - phytoliths are three-dimensional bodies that can look very different depending on the angle of view. Mounting phytoliths in liquid media (e.g. microscopy immersion oil) will help understanding the 3D appearance and enhance the chances of identification.
- Short cells phytoliths from grasses (Poaceae) tend to have a small bubble of cytoplasm in their body. This characteristic makes the identification of these phytoliths rather straightforward.
This set of characteristics should help in correctly identifying a phytolith. However, sometime volcanic ash particles can be confused for phytoliths. These particles are a common component in soils and sediments in areas where there are or there have been active volcanoes.
See the following illustrations to train your eye on real and fake phytoliths.