anorthosite - teaching student specimens UNIT OF 5 SPECIMENS
This unusual igneous rock superficially resembles granite, differs from granite in that it is composed entirely of plagioclase feldspar - essentially a granite minus the quartz, hornblende or mica. In this case the type of plagioclase is andesine, a calcium sodium feldspar with the ratio of sodium to calcium about 7:3. It is Middle Proterozoic in age.
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.
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