SCANNING ELECTRON MICROSCOPY OF LITHOPHYSAE AND 3-D VISUALIZATION OF THE INTERNAL STRUCTURE OF BANDED AGATE NODULES

Lithophysae, commonly referred to as “thundereggs,” are nodule-like mineral structures found only in rhyolite or perlite lava flows or domes. As relatively little is known about the formational processes of these structures, the purpose of my research is to contribute to a greater, collective understanding of the chemical and physical processes responsible for the formation of lithophysae. Through scanning electron microscopy (SEM), both compositional and physical properties across the bands of agate are analyzed and combined SE/BSE photomicrographs are taken. 3-D visualization is achieved by taking high-quality photographs of both faces of several slices cut from a single sample and digitally tracing individual bands. These same 2-D bands are correlated between each image. Using a Python script, each pixel along these traced contours is extracted and translated into coordinate points to be viewed in a 3-D point cloud viewer. Each quasi-circular 2-D band traced on the image represents a portion of a quasi-spherical 3-D layer within the agate. By correlating similar bands it is possible to reconstruct the “sphere.” A mesh is created to make these contour lines continuous and enhance visualization capabilities. The SEM results show negligible compositional variations between the bands; all layers are >98 wt% silica. However, distinct physical differences are evident as alternating topography (developed during polishing). I interpret these outcomes to mean that differences in crystallization are responsible for the both the subtle differences in hardness and orientation of the crystals that account for the topography of the agate. Degassing in the center of the nodule opens the cavity, which are filled with silica. Subsequent degassing allows for flow structures to form. I interpret the banding to mean that solidification begins on the wall of the hollow cavity, and bands are continuously forming as infilling occurs. 3-D visualization allows for a stratigraphy to be determined. Cross-cutting relationships of some degassing features and banding indicate that the silica flowed and, thus, was likely a gel-like ductile substance initially, before complete crystallization occurred.