Kentucky Agate: State Rock and Mineral Treasure of the Commonwealth

Chapter 5. Microscopic Petrgraphy of Kentucky Agate

39 5 Microscopic Petrography of Kentucky Agate A Microscopic Look at Agates in Reflected and Transmitted Light Normally, to an unaided eye, the color bands in Kentucky agate look multicolored, and the hues—whether red, yellow, black, or white—appear to be solid colors. But when the same agates are examined with optical petrography through a powerful microscope, a previously undetected world of iron oxide pellets, globules, and spheres illuminating the bands can be seen to form the various colors. Iron minerals such as hematite and limonite are very common, and their oxidation produces the variance in colors we see in agates. The photographs in this section show unique agates and agate banding. The rock samples have been cut and polished for viewing in reflected light. When observed under microscopes at various magnifications, components of the rocks are visible in both transmitted light and reflected light. Optical petrographic photography was conducted with an Olympus BX-51 petrographic stereo microscope utilizing an Olympus DP-71 optical computerized camera. The photographs were captured at various exposure times through a 2x, 4x, 10x, or 20x objective lens on the Olympus microscope. ky agate.indd 39 7/27/13 4:48 PM Figure 10. Heart-shaped banding showing limonite (yellow) and hematite (red) spherule inclusions. Figure 11. Red spherulitic hematite bands underlain by clear quartz overgrowths on yellow limonite banding. 40 ky agate.indd 40 7/27/13 4:48 PM Figure 12. Translucent chalcedony showing inclusions of red hematite spherules. Figure 13. Red hematite fillings in fractures in agate. 41 ky agate.indd 41 7/27/13 4:48 PM Figure 14. Translucent and botryoidal chalcedony showing a circular form of red hematite inclusions. Figure 15. Symmetrical dark quartz overgrowths separated by yellow limonite banded inclusions. 42 ky agate.indd 42 7/27/13 4:48 PM Figure 16. A large floating quartz crystal surrounded by flowing yellow limonitic globules containing a crown of red hematite. Figure 17. The color change in agate bands reflects a change in oxidation state. 43 ky agate.indd 43 7/27/13 4:48 PM Figure 18. The color change in this agate represents changes in iron mineral oxidation states. Figure 19. Hematite oxidation. 44 ky agate.indd 44 7/27/13 4:48 PM Microscopic Petrography of Kentucky Agate 45 Transmitted- and Reflected-Light Photographs The following photomicrographs are of polished thin sections of agate. Images were photographed with transmitted and reflected light as seen through the Olympus microscope. The transmitted light shows the chalcedonic nature of some of the quartz by featuring its highly fibrous structure, and reflected light shows the natural color and appearance of the agate. The reflected-light photos of the translucent chalcedony give a view into the agates and their spherule inclusions of iron oxide minerals and other sedimentary textures. Figure 20. This transmitted-light image of an agate shows black iron oxide inclusions and the pleochroic fibrous texture of chalcedonic quartz. ky agate.indd 45 7/27/13 4:48 PM Figure 22. This transmitted-light image of the agate shown in figure 21 shows the chalcedonic fibrous nature of the quartz bands in the upper left portion of the photograph and the coloring of the megaquartz crystals. 46 Figure 21. This reflected-light image shows the thousands of spherule inclusions that make up the red and yellow banding. ky agate.indd 46 7/27/13 4:48 PM Figure 24. This transmitted light image of the agate shown in figure 23 shows two concentric spherule bands with black hematite inclusions overlain by chalcedonic quartz fibers in the banding. 47 Figure 23. This reflected-light image shows concentric spherule bands with red hematite inclusions. ky agate.indd 47 7/27/13 4:48 PM 48 Figure 25. Transmitted-light photograph of dolomite crystals that have been replaced by chalcedony containing inclusions of black opaque hematite crystals. ky agate.indd 48 7/27/13 4:48 PM ...