orbicular granite (monzonite) - hand specimen of a very unusual plutonic rock from Peru

Here's a really rare rock. Orbicular granite is often not a true granite - this example has a quartz monzonite matrix. Monzonite is a relatively uncommon rock, usually forming only a part of granitic plutons. These specimens were collected from a quarry near Asuntamiento El Quirio about 4 miles NNW of Lurigancho-Chosica in Lima Province, Peru.

Like the orbicular granite quarry at Boogardie Station in Western Australia, this quarry has been purchased by Chinese interests. The Australian quarry is now fenced off with no access. No material is coming out of either locality, unfortunately.

The dark crystals of the orbicules are bytownite, a calcium-rich feldspar. As the orbicules crystallized, they appear to have settled by gravity while still a skeletal mesh of crystals, since they are frequently deformed or molded against one another. Interestingly, since it only shows up in reflectance infrared spectroscopy, this plutonic rock was infiltrated by copper-bearing solutions. 

These 1/4" slabs are cut but not polished. Both sides of the first one are shown.

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