Contributions to mineralogy and petrology vol:130 issue:3-4 pages:320-335
Quantitative X-ray texture analysis was used to identify the fiber elongation direction of microfibrous quartz varieties in materials that contain narrow bands of closely spaced, radially divergent spherulitic fiber bundles. The statistical approach of X-ray texture analysis makes it possible to determine the fiber axis direction even when the maximum divergence angle of fibers within spherulitic fiber bundles diverge +/-20 degrees from a common growth direction. Representative samples of chalcedony [11 (2) over bar 0], quartzine , and pseudochalcedony were selected for additional study with transmission electron microscopy. All varieties of microfibrous quartz were found to contain high concentrations of polysynthetic Brazil twins and domains characterized by a modulated microstructure. Trace element profiles of aluminum and iron concentration were measured by electron microprobe analysis for an agate sample that contains alternating bands of chalcedony [11 (2) over bar 0] and quartzine fibers. The elemental concentration profiles were measured in a direction parallel to the growth direction of microfibrous quartz. Although fibers within the chalcedony bands initially displayed cooperative helical twisting, the fibers became untwisted during the later stages of growth within any one band. The trace element profiles revealed that higher concentrations of aluminum are associated with quartzine and non-twisted [11 (2) over bar 0] fibers, rather than twisted chalcedony [11 (2) over bar 0] fibers. Except for the presence of iron-rich inclusions between quartzine and chalcedony bands, trace concentrations of iron were below the detection limit for the different varieties of microfibrous quartz analyzed.