Transmitted Circular Polarized Light Illumination
Definition/Function:
Phytoliths are mineral deposits formed by plant tissue. They may be hydrated silicon dioxide (opal), calcium oxalate monohydrate
calcium oxalate dihydrate, calcium phosphate, or calcium carbonate. These structures have distinctive shapes and often can
help identify the plant of origin when found free in an environmental sample. They are very common airborne particles in
arid environments and were identified in the dust captured on the sails of the HMS Beagle in 1833, as reported by Charles
Darwin. The calcium oxalate phytoliths from cacti contribute to the calcareous aerosols of the Southwest United States.
The characterization of phytoliths and the identification of their plant of origin is progressing as a result of their
value in determining ancient environments from soil samples. They have also been used to identify the beginnings of
agriculture at archeological sites. These studies all fall under the general discipline of Palaeobotany. In palaeobotany
the term is generally limited to the silicon dioxide type of phytolith because they are the most persistent in the
environment.
Significance in the Environment:
These particles are left behind when plant materials degrade or are burned. The siliceous phytoliths typically become
amorphous, transparent particles of distinctive shape. When burned they often become coated with a layer of carbon and
appear black or gray. Calcareous phytoliths may remain intact as the plant degrades but they often go through a series of
chemical reactions that ultimately result in the formation of a calcium carbonate. When burned they convert to calcium
oxide, which then reacts with water and carbon dioxide to form aragonite (calcium carbonate). Cubical calcium oxide and hydroxide
particles are common in the plume from the combustion of wood, often showing surface modification to the carbonate. The
surface modification is evident as a birefringent film over part of the particle.
Calcium oxalate phytoliths are not consumed by fire so they become flyash if this
sawdust is disposed of by burning. Sawdust has historically been used as a fuel providing
power and heat at lumber mills or sold as "hog-fuel" for furnaces or boilers designed to
burn any inexpensive or available combustable waste as its fuel. The calcium oxaltate
becomes calcium oxide in the heat of the combustion chamber and becomes pseudo-cubes of
flyash. Calcium oxide pseudo-cubes react with water vapor, sulfur oxide gases, and carbon
dioxide in the plume to form anisotropic compounds on the surface of the crystal. These
pseudo-cubes retain their shape but become mottled in appearance when view with polarized
light. When seen in an outdoor air sample they are a sure indicator of a hog-fuel boiler.
Characteristic Features:
The silica phytoliths are characterized by their low refractive index, less than 1.48, their cell related morphology (see Madella,
Alexandre, and Ball; 2005), and by their lack of birefringence. The other phytoliths are dominated by their crystalline structure
rather than cell morphology. The crystalline habit manifested by these phytoliths will vary from plant species to plant species
and from cell type to cell type within a species. The two types of calcium oxalate phytoliths from the leaves of Rhubarb are
an example of variation between cell types in a single species.
Associated Particles:
References:
1. Blinnikov, Mikhail, "Phytoliths in plants and soils of the interior Pacific Northwest, USA", REVIEW OF PALAEOBOTANY &
PALYNOLOGY, vol. 135, pp. 71-98, 2005
2. Franceschi, Vincent R. and Harry T. Horner Jr., "Calcium oxalate crystals in plants", THE BOTANICAL REVIEW, vol. 46, No. 4,
Oct-Dec 1980, pp. 361-427.
3. Piperno, Dolores R., PHYTOLITHS, AltaMira Press, 2006.
4. Rapp, George Jr. and Susan C. Mulholland (eds), PHYTOLITH SYSTEMATICS, Plenum Press, 1992.
5. Madella, M., A. Alexandre, and T Ball, "International Code for Phytolith nomenclature 1.0", ANNALS OF BOTANY, 2005,available
on line at http://aob.oxfordjournals.org/cgi/reprint/mci172v1
6. http://en.wikipedia.org/wiki/Phytoliths
