The mineralogy of partially kaolinized strata interbedded with lignite at the San Miguel mine, Atascosa County, Texas, was investigated by X-ray powder diffraction and optical and scanning electron microscopy. The San Miguel lignite occurs in the lower Jackson Group (late Eocene) of southern Texas. Based on mineralogical and micromorphological data, some of these clay partings are probably volcanic in origin and were exposed to variable degrees of in situ kaolinization in a swamp environment. Coexistence of kaolinite, clinoptilolite, and opal-CT in several of these strata suggests that the partially kaolinized volcanic layers were subjected to a subsequent resilication process following burial. Kaolinite is the dominant mineral in the oldest and most kaolinized volcanic layer (underclay) below the lowest lignite bed (seam D). The kaolinite exhibits a well-developed vermicular morphology. The youngest volcanic layer, which occurs stratigraphically above the uppermost lignite seam, is characterized by pseudomorphs of volcanic glass shards and consists mainly of clinoptilolite. Movement of siliceous ground water from this layer to the underlying strata apparently provided silica-rich solutions from which opal-CT and large (as long as 300 μm) euhedral crystals of clinoptilolite precipitated in the fossilized plant roots, veinlets, and fractures within the underlying strata. Micromorphological relationships between the Sirich (opal-CT and clinoptilolite) and sulfide (marcasite and pyrite) minerals in the fossil roots and fractures suggest that the marcasite formed before and pyrite after the resilication process.

The use of in vivo X-ray microcomputed tomography (μCT) to study plant root systems has become routine, but is often hampered by poor contrast between roots, soil, soil water, and soil organic matter. In clinical radiology, imaging of poorly contrasting regions is frequently aided by the use of radio-opaque contrast media. In this study, we present evidence for the utility of iodinated contrast media (ICM) in the study of plant root systems using μCT. Different dilutions of an ionic and nonionic ICM (Gastrografin 370 and Niopam 300) were perfused into the aerial vasculature of juvenile pea plants via a leaf flap (Pisum sativum). The root systems were imaged via μCT, and a variety of image-processing approaches used to quantify and compare the magnitude of the contrast enhancement between different regions. Though the treatment did not appear to significantly aid extraction of full root system architectures from the surrounding soil, it did allow the xylem and phloem units of seminal roots and the vascular morphology within rhizobial nodules to be clearly visualized. The nonionic, low-osmolality contrast agent Niopam appeared to be well tolerated by the plant, whereas Gastrografin showed evidence of toxicity. In summary, the use of iodine-based contrast media allows usually poorly contrasting root structures to be visualized nondestructively using X-ray μCT. In particular, the vascular structures of roots and rhizobial nodules can be clearly visualized in situ.