ÒBLACK GRANITESÓ
The Òblack granitesÓ are popular and attractive
decorative stones. However, the term Òblack graniteÓ makes a geologist
cringe. These rocks are anything but granite. Granites are felsic
intrusive igneous rocks, and as such are light-colored. Dark-colored
igneous rocks generally have a mafic chemistry, as do all the rocks shown
below. Such rocks are actually gabbros, norites, gabbronorites, and
dolerites. They are dominated by the minerals plagioclase feldspar and
pyroxene.
Black Pearl Granite - a Proterozoic-aged gabbro from Guntur District,
eastern Andhra Pradesh State, southeastern India.
Black Cambrian Granite - a Precambrian norite from Quebec, Canada.
Geologic unit & age:
Lac-St.-Jean Anorthosite Suite, late Mesoproterozoic,
1.14 to 1.16 b.y.
Locality:
quarry at St. Nazaire, Chicoutimi County, southern Quebec, southeastern Canada.
Black Galaxy Granite - a Proterozoic-aged norite from India's Eastern
Ghats Orogenic Belt. This is probably the most popular and attractive
black granite available in the commercial decorative stone trade. It has
blackish plagioclase feldspar and bronzite pyroxene crystals.
Bronzite is an orthopyroxene and it has a spectacular copper-colored play of
color on cut & polished surfaces (even on broken surfaces). The
result is a rock surface reminiscent of sparkling stars in space, hence the
commercial name ÒBlack GalaxyÓ. This rock is quarried in the Chimkurthi
area, Prakasam District, Andhra Pradesh State, southern India.
Impala Black Granite - an attractive, 2 billion year old gabbronorite from
South Africa. It is composed principally of grayish plagioclase feldspar
and black pyroxene. It comes from the famous Bushveld Complex, a
world-class example of an LLI (large layered igneous province). LLIs
often have economic concentrations of valuable metals, such as platinum and
chromium. North America has one economically significant LLI - the
Stillwater Complex of Montana.
The Bushveld has a thick stratigraphy that is well
documented. The gabbronorite sample shown above comes from the upper Main
Zone of the Rustenburg Layered Suite. It dates to the mid-Paleoproterozoic
(2054-2061 million years). ÒImpala Black GraniteÓ is quarried north of
the town of Rustenburg in northeastern North West Province (= northwest of the
city of Johannesburg), northeastern South Africa.
Indian Absolute Black Granite - the name ÒAbsolute Black GraniteÓ has been applied
to relatively finely-crystalline mafic intrusive igneous rocks from many
localities on different continents (e.g., Africa, India, China). This
sample is from a large Precambrian dike of dolerite (a.k.a. diabase; a.k.a.
microgabbro) that cuts through igneous & metamorphic rocks of southern
India's Dharwar Craton.