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Pyrolyzed Calcium Oxalate Phytoliths Under the Microscope

Wildfire Assemblage

This is from an environmental tapelift collected outside a home that was near a large Colorado wildfire. This one field of view contains pyrolized calcium oxalate phytoliths, ash, burnt clay and charred woody shrub particles. The reflective, opaque fibers on the left are from a woody shrub. The white crystal shapes are polycrystalline polymorphs of calcium oxalate phytoliths. The clear crystals at right center are calcium oxide polymorphs of calcium oxalate phytoliths. The whitish clouds are ash consisting of potassium, sodium, magnesium, and calcium oxides. The reddish particles are natural minerals coated with clay containing iron that has been oxidized to hematite (Fe3O4). This field of view indicates a very hot fire (the clear phytoliths), burning deciduous trees (all crystalline phytoliths), burning woody shrubs (char on left), high convective winds (burnt clay), and ash from biomass combustion. 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 and Reflected Darkfield Illumination


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: