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 ÒmetanorthositeÓ), 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.
