What are the dark minerals?
A close inspection reveals that the boulders contain dark gray crystals up to 2 inches long that reflect the sun’s rays (Figure 3). 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. Many of them show signs of twinning, in which two halves of the crystal are mis-aligned, but joined in the middle. The twins can be recognized by the way light reflects from only one of the twins at a time (Figure 4). This rock is composed of several other minerals, including dark pyroxenes (augite and clinopyroxene) and amphiboles (hornblende and actinolite) (King and Foord, 1994; West and others, 2000).
