Figure 7-9. A: Fine red-brown layers in columnar calcite in a stalagmite. Although the color of the bands defining these layers is suggestive of iron oxide, energy-dispersive microprobe analysis reveals the presence of only Mg and Si and perhaps Al, suggesting the presence of a smectitic clay mineral. Field of view is 2.4 mm wide. B: High-magnification view of area in red rectangle in A. Note the asymmetry of these layers, in that clear calcite grades upward into cloudier calcite that makes an abrupt transition upward to clear calcite. Layers are about 40 microns (0.04 mm) thick; field of view is 0.06 mm wide. Both photomicrographs were taken in plane-polarized light.      De Soto Caverns, Alabama, U.S.A; Stalagmite 95a; thin section 95a4. Sample provided by Dr. George A. Brook.
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