ANORTHOSITES

 

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.

 


 

Blue Eyes Granite - a coarsely-crystalline anorthosite from Labrador.  All of the lightish to darkish gray material in this rock is labradorite - every crystal will flash bright blue when tilted at the correct angle in the light.  This expensive decorative stone comes from the Ten Mile Bay Quarry, near the town of Nain along the Labrador coast, eastern Canada.  The quarry exploits the Nain Anorthosite (Nain Plutonic Suite), a mid-Mesoproterozoic intrusion (1.29 to 1.35 billion years) emplaced along the Abloviak Shear Zone.

 


 

Labrador Antique Granite - a brownish, coarsely-crystalline labradoritic anorthosite from Scandinavia.  Each brown crystal will flash bright electric blue if the rock is tilted at the correct angle.  Despite the name, this rock does not come from Canada.  It comes from the Rogaland Anorthosite Complex of southwestern Norway.  The Rogaland Anorthosite Complex is a late Mesoproterozoic intrusion that dates to about 929-932 million years.  It is quarried near the town of Egersund in southern coastal Rogaland County, far-southwestern Norway.

 


 

Mountain Green Granite - a greenish garnetiferous metanorthosite from upstate New York’s Adirondack Mountains, USA.  This is from the Marcy Anorthosite, which was emplaced 1.155 billion years ago, during the Mesoproterozoic.  The sample shown above comes from the Cold Springs Granite Quarry, off Rt. 9N between the small towns of Au Sable Forks and Jay, northeast of Lake Placid.  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 Spring Granite Quarry produces garnetiferous metanorthosite in seven color grades (1 being light-colored and 7 being dark-colored).  The sample shown above is color grade 6.

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

 


 

Volga Blue Granite (above &  below) - a dark, very coarsely-crystalline anorthosite with zoned plagioclase feldspar from Ukraine.  This is from the Volodarsk-Volhynsky Intrusion in the southern Korosten Complex, a large igneous suite intruded through the Ukrainian Shield.  The Volodarsk-Volynsky Intrusion dates to the late Paleoproterozoic (1.758-1.760 Ga).  This material is quarried between the cities of Korosten and Zhitomir, central Zhitomir Province, northwestern Ukraine.

 


 

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