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Typical Minerals
The rock anorthosite is named after the mineral anorthite - Calcium rich plagioclase. Anorthosite occasionally has pyroxene associated with it, but less than 10%.
     Anorthosite is a very unusual rock, not only because it is composed only of calcium plagiclase, but also because it is relatively rare, and forms by mechanisms we do not fully understand.
     Anorthosite is common on the moon, however, composing much of the rugged upland areas (not the smooth mare.) It is likely that when the earth formed anorthosite was one of the more common constituents, just because calcium is a common element in interstellar gas clouds, and planetary nebulae.
     This specimen was collected from a road cut that was about 50 feet high and a couple of hundred meters long, and it was all anorthosite just like this. In fact, this rock could be seen in the canyon walls for miles around.
     Typically when we find anorthosite it is in large masses covering hundreds of square miles at least.
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Tectonic Association
      Although anorthosite was probably common in the early history of the earth it has become less and less common through time. Little if any is forming today in places we can see. Anorthosite seems to be associated with rifting events, but there is a good liklihood that more than one mechanism is responsible for its formation.
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