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
Definition/Function:
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:
References:
http://univ-provence.academia.edu/JacquesBrochier/Papers/198408/Calcite_Crystals_Starch_Grains_Aggregates_or..._POCC_Comment_on_Calcite_Crystals_Inside_Archaeological_Plant_Tissues
