File:Mountain Green Granite (garnetiferous metanorthosite, Marcy Anorthosite, Mesoproterozoic, 1.155 Ga; Cold Springs Granite Quarry, near Jay, Adirondacks Mountains, New York State).jpg

English: "Mountain Green Granite" - metanorthosite from the Precambrian of New York State, USA.

Anorthosites are uncommon intrusive igneous rocks almost exclusively composed of Ca-rich plagioclase feldspar. There’s usually a blackish pyroxene component as well. Anorthosites having labradorite plagioclase feldspar will display a wonderfully colorful iridescent play of colors (labradorescence). This makes them desirable decorative stones.

This greenish, garnetiferous metanorthosite is a polished decorative stone from the Marcy Anorthosite, which was emplaced 1.155 billion years ago, during the Mesoproterozoic. The rock comes from the Cold Springs Granite Quarry, off Rt. 9N between the small towns of Au Sable Forks and Jay, northeast of Lake Placid in the Adirondack Mountains of upstate New York. Anorthosite is an uncommon rock on Earth, but is relatively common in the Adirondacks. Adirondacks anorthosite has been metamorphosed (hence “metanorthosite”), and in many places the rocks have blackish streaks representing foliation. The rock is dominated by slighly labradorescent calcic plagioclase feldspar (greenish gray) with pyroxene (black) and garnet (very dark red).

In 2004, at Ground Zero in downtown Manhattan, a 20-ton block of garnetiferous metanorthosite from this quarry was laid as the first cornerstone of the Freedom Tower. The Cold Springs Granite Quarry produces garnetiferous metanorthosite in seven color grades (1 being light-colored and 7 being dark-colored). The sample shown here is color grade 6.

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Quarry access, samples, and some info. provided by Rick Barber, manager of the Cold Springs Granite Quarry at Jay, New York.