Sawdust With Phytoliths
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 distictive 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.

Significance in the Environment:
These particles are left behind when plant materials degrade or are burned. The silaceous phytoliths typically become amorphous, transparent particles of distictive shape. When burned they often become coated with a layer of carbon and apear black or gray. Calcareous phytoliths may remain intact as the plant degrades but they often go through a series of chemical reactions that ultamately 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.

This fragment of sawdust contains numerous pseudo-cubical crystals of calcium oxalate. These calcium oxalate phytoliths are not consumed by fire and 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. With circular polarized light all anisotropic materials exhibit their maximum interference color regardless of the orientation of the stage so no parts of the sawdust particle or the calcium oxalate phytoliths are in or near an extinction position.

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References:
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. Piperno, Dolores R., PHYTOLITHS, AltaMira Press, 2006. Rapp, George Jr. and Susan C. Mulholland (eds), PHYTOLITH SYSTEMATICS, Plenum Press, 1992.