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 charred
fibers on the left are from a woody shrub. The crystal shapes with bright spots and
dark spots are both polycrystalline polymorphs of
calcium oxalate phytoliths. The clear crystals at right center are calcium oxide
polymorphs of calcium oxalate phytoliths. The textured
colorless background and the clouds of small crystals are ash consisting of
potassium, sodium, magnesium, and calcium oxides. The dark
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 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
