PERIDOTITE
Peridotites are coarsely-crystalline, ultramafic, intrusive igneous rocks.
Igneous rocks have an ultramafic chemistry if they are <45% silica (=
SiO2 chemistry) and are dominated by the minerals olivine and/or
pyroxene. They are often colored greenish, greenish-black, or
brownish-black.
Peridotites are scarce in much of the Earth’s
crust, but are common in Earth's upper mantle and basal crust. There are
a few places on Earth where mantle rocks & basal crustal rocks have been
uplifted to the surface. Examples of localities with exposed mantle
peridotites (or metamorphosed peridotites) include Oman, Cyprus, the Shetland
Islands, Austria, and Newfoundland. Sometimes, peridotite rocks from the
mantle get caught up in rising masses of magma. When erupted from
volcanoes at the surface, the lava flows will have mantle peridotite xenoliths.
There are seven principal varieties of peridotite,
based on different percentages of the three main minerals: olivine,
clinopyroxene, and orthopyroxene. These seven rocks are dunite,
pyroxenite (formerly websterite), clinopyroxenite, orthopyroxenite,
harzburgite (formerly saxonite), wehrlite, and lherzolite. Dunite
is >90% olivine, with minor pyroxene. Pyroxenite is >90%
pyroxene, with minor olivine. If a pyroxenite is >90% composed of a
calcium-bearing pyroxene (clinopyroxene, such as augite or diopside), the rock
is called clinopyroxenite. If a pyroxenite is >90% composed of
a calcium-free pyroxene (orthopyroxene, such as enstatite or bronzite), the
rock is called orthopyroxenite.
Lherzolite, harzburgite, and wehrlite have decent
mixes of olivine & pyroxene. Harzburgite is a mix of olivine
and orthopyroxene (both >10% of the rock). Wehrlite is a mix of
olivine and clinopyroxene (both >10% of the rock). Lherzolite
is a mix of olivine and orthopyroxene and clinopyroxene.
Three varieties of peridotite are shown below.
Dunite
- a peridotite here composed ~exclusively of olivine. This sample is about 99% forsterite
olivine crystals (= green) and
about 1% chromite crystals (= black).
Pyroxenite - a peridotite exclusively composed of blackish-green pyroxene. This sample is from the 2.71 billion year old Stillwater Complex of Montana, USA.
Harzburgite - a peridotite consisting of a mix of olivine & pyroxene,
resulting in a brownish-black
colored igneous rock. This sample is from the 2.71 billion year old Stillwater
Complex of Montana, USA.