This unusual igneous rock superficially resembles granite, differs from granite in that it is composed almost entirely of the plagioclase feldspar anorthite - essentially a granite minus the quartz, hornblende or mica, though there is minor muscovite. In this case, the type of plagioclase is anorthite, the most calcium rich and sodium poor member of the plagioclase group, with a Ca: Na ratio of 90-100:10-0 . It is middle Proterozoic in age. The Apollo astronauts brought a very similar anorthosite back from the moon.
This anorthosite was intruded into a previously metamorphosed gneiss about 1.2 billion years ago, forming a syenite-anorthosite body at least 10 km in thickness and 15 km in diameter. The anorthosite body was formed as an accumulation of plagioclase crystals at the bottom of the magma chamber.
Anorthosite can vary widely in color. An anorthosite complex in Wyoming yields anorthosite that is very dark gray, almost black. It occurs in eastern Canada, in the Adirondack Mtns. of New York, and on the moon. This anothosite is similar to the samples the Apollo 15 astronauts brought back from the moon, though slightly chemically different.
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