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Goethite with Calcite Ore
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Goethite with calcite. We are working with reflected light, using the medium magnification lens and the maximum zoom of the camera. In the image we have a material that probably filled a basalt rock cavity during a lava flow. It was found loose, in the middle of the detonated material and is composed of goethite, which are these white balls that we are seeing now, carbonate, calcite, which is the gray material and the clay-minerals that are the darkest colors, sometimes yellow as the left in the image.
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Thus, clay minerals, calcite and goethite are characteristic secondary minerals that occur in basaltic rocks. Here goethite does not occur in amethyst, although it is relatively common. It is easy to see radial aggregates of acicular crystals of goethite in amethyst, but here goethite occurs in calcite. Then you can form these white balls, as in the center of the image. Or, sometimes, you still see goethite needles sticking out of these spheres.
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Looking at this occurrence with the objective of maximum expansion, we have this situation here. To see better, let's decrease the brightness a little. Now, these radial aggregates are clearly visible with these protruding needles, coming out of the aggregates.
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In crossed nicolls, we have the typical characteristics of goethite, which are these red reflections associated with an anisotropy in gray. When the needles are well separated from each other, these golden colors can be seen, which even look like rutile. In some cases, these Goethite needles have been mistaken for rutile, because it also occurs in quartz, rutilated quartz is common.
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However, occurring in the amethyst of basaltic rocks, these needles are made of goethite, remembering that here goethite is in calcite, which is a rarer situation, but the habit is the same. Spherical aggregates, sometimes with needles sticking out, showing the golden tones typical of Goethita. And in the aggregates it is difficult to see, because the crystals are very small, but it is still possible to see a little of the anisotropy in gray and the reddish goethite.
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