Figure 3. Close inspection of the sample shown in Figure 2 reveals that the boulders contain dark gray crystals up to 2 inches long that reflect the sun's rays. These large crystals are minerals in the feldspar family, namely the alkali feldspars orthoclase and microcline. Alkali feldspars are usually light (white or pink) not dark, as in this rock. In earlier work Perkins (1922) misidentified the microcline as labradorite, a plagioclase feldspar that is commonly blue-gray in color. The crystals have sharp edges.

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Last updated on October 6, 2005

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