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Figure 3-2. Chalecdony, a form of silica (S), in a thin layer over aragonite (A) and below and above calcite (C) in a stalagmite. The black-and-white (to slightly yellow) birefingence is typical of silica minerals; the fibrous crystals visible here are a defining feature of chalcedony . A corresponding figure shows the same field of view in plane-polarized light. A higher-magnification view in cross-polarized light shows the fibrous crystals in greater detail. Photomicrograph was taken in cross-polarized light; field of view is 0.8 mm wide. Carlsbad Caverns, New Mexico, U.S.A; Georgia Giant stalagmite; thin section CB64. |
