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Figure 3-5. A silica stalagmite consisting of generally randomly oriented chalcedony (A), a thick layer of large bundles of chalcedony (B), and a finely laminated zone of small bundles of chalcedony (C), all of which are visible in a corresponding view in cross-polarized light. However, none of that is visible here, which is typical of views of silica rocks in plane-polarized light. The blue arrow points to one of the laminae in the small bundles. The purple arrows point to strikingly straight portions of the upper boundary of the randomly oriented chalcedony (Zone A), which would be compatible with speculation that the randomly oriented chalcedony resulted from silicification of a pre-existing calcite layer.
Photomicrograph was taken in plane-polarized light; field of view is 2.3 mm wide. Gaalweyte Cave, Somalia; Stalagmite GG2; thin section GG2F. Sample collected by Dr. George A. Brook. |
