Calcium Oxalate Phytoliths in Environmental Samples

Calcium oxalate phytoliths, most commonly whewellite or calcium oxalate monohydrate (CaC2O4·H2O), are a regular part of environmental samples, though generally at low concentrations. They tend to be misidentified as calcite (CaCO3) in environmental samples due to their very high birefringence (0.160) and similar refractive indices (α = 1.490, β = 1.555, and γ = 1.650). Also, being that typical environmental samples are analyzed in a fixed mounting medium, with no ability to roll the particles individually or change refractive index media easily, leads to their misidentification. The crystal habits of calcium oxalate phytoliths are unique, which helps differentiate them from environmental calcite. The crystal habit of a phytolith is the result of chemicals in the cells that promote the growth of specific faces. The result is that the habits available to specific plants are under genetic control to some extent. Some of these faces readily form twinned crystals, which also seems to be under some genetic control. These twins add to the diversity of unique crystal habits. They can be grouped into five general habits: laths, styloids, prisms, druses, and sands. Styloids, prisms, and druses are the most diagnostic of the plant from which they originate. This paper describes the calcium oxalate phytoliths of eight plants: Pseudotsuga menziesii (Douglas fir), Larix occidentalis (western larch), Pinus ponderosa (ponderosa pine), Quercus turbinella (shrub live oak), Rosa nutkana (wild rose), Larrea tridentata (creosote bush), Cercocarpus intricatus (mountain mahogany), and Adenostoma sparsifolium (redshanks). The optical and crystallographic properties of these phytoliths are included with photomicrographs taken with circularly polarized light (CPL) so that all the crystals in the microscope field of view show characteristic interference colors. The setup for CPL and a sodium hypochlorite digestion procedure (wet ashing) for the generation of reference collections are briefly explained in this paper. How calcium oxalate crystals appear in an environmental sample and how to prepare reference materials as needed are also discussed.