Druss Form of a Calcium Oxalate Phytolith from Rhubarb
This rosette crystal mass (druss form) is from the leaf of a rhubarb plant, Rheum rhaponticum. This is one
of two forms of calcium oxalate phytoliths common in the leaf. The other form is a single
prismatic crystal. This phytolith was collected by digesting a rhubarb leaf in sodium hypochlorite solution (Bleach).
Transmitted Circular Polarized Light Illumination
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 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. When the plant containing calcium oxalate phytoliths is burned
the phytoliths go through a series of chemical reactions. First they begin to loose the waters of hydration. That begins at about 120 degrees
Celsius. Next, carbon monoxide is released and calcium carbonate begins to form on the surface of the crystal. That begins at a temperature of about
420 degrees Celsius. At this point the crystal generally still maintains its original shape. The crystal shows the effect of the exposure to heat
but the shape is still consistent with that characteristic of the original plant. Continued heating ultimately result in the formation of a calcium
oxide, begining at about 620 Celsius. 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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Oct-Dec 1980, pp. 361-427.
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4. Rapp, George Jr. and Susan C. Mulholland (eds), PHYTOLITH SYSTEMATICS, Plenum Press, 1992.
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on line at http://aob.oxfordjournals.org/cgi/reprint/mci172v1