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Pyrolyzed Calcium Oxalate Phytoliths Under the Microscope

Pyrolyzed Calcium Oxalate Phytolith

This is from an environmental tapelift collected outside a home that was near a large Colorado wildfire. The calcium oxalate phytoliths at upper right are still identifiable by their shape but they have transformed as a result of the heat of the fire into calcium oxide (CaO). The phytoliths at lower left still have many areas that persist as calcium carbonate (CaCO4). The calcium carbonate crystals are within the particle rather than on the outside. When calcium carbonate forms on the outside it is the result of the calcium oxide reacting with carbon dioxide and water in the air. The shape of these phytoliths are consistent with deciduous trees, such as Gambel and shrub live oak, and not conifers.

Transmitted Off Crossed Circular Polarized Light


Calcium oxalate phytoliths are present in two chemical forms and in many different crystalline habits in plant material. The two chemical forms are CaC2O4-H2O, whewellite, and CaC2O4-H2O, weddellite. Calcium oxalate phytoliths are exposed to high temperature, water vapor, and carbon dioxide in the plume of a fire. Calcium oxalate converts directly to microcrystalline calcite in the temperature range of 430 to 510 degrees Celsius. If temperatures are higher then calcium oxalate will convert to calcium oxide and then may react with water and carbon dioxide to form calcium carbonate. All of these reactions are a function of time at temperature. If the time is not sufficient at high enough temperature the reaction may not be complete. The reactions begin at the surface and move inward.

Significance in the Environment:

Characteristic Features:

Weddellite is an optically positive tetragonal crystal with refractive indices of 1.523 (w) and 1.544 (e), for a birefringence of 0.021. Whewellite is an optically positive monoclinic crystal with refractive indices of 1.490 (a), 1.554 (b), and 1.650 (g), for a birefringence of 0.160. The pyrolysis products of both tend to be calcium carbonate. The calcium carbonate poly-crystalline aggregate may retain the structure of the original calcium oxalate crystal but the form of the individual calcium carbonate crystals making up that shape may be as aragonite or vaderite tablets or spherulites.

Associated Particles: