orbicular granite (monzonite) - a very unusual plutonic rock from Peru - uneven thickness end cut

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 is now closed. 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. 

An uneven thickness end cut from a block of this material. Both sides are shown.

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