Calcium Oxalate Phytolith in a Leaf Fragment
Transmitted Circular Polarized Light

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.

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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.