Three important
igneous rocks for our study of the Moon:
(These will also be
important in our study of the other inner planets as well as the meteors and
meteorites.)
1. Extrusive
(Volcanic)
Basalt
An extrusive igneous rock. Basalts crystallize from cooling
lavas that flow from fissure eruptions and shield volcanoes. Because the
cooling is fairly rapid, the crystals never get very large—the mineral grains
are too fine to be seen without magnification. Basalts are made largely of the
minerals plagioclase and pyroxene. These tend to be darker minerals and basalts
in general are dark in color. On a fresh surface, color often ranges from dark
gray to black. Because of the iron in the constituent minerals oxidizes in with
exposure to air and moisture, weathered surfaces can be tan, brown or reddish
brown.
2. Intrusive
(Plutonic)
Anorthosite
An intrusive igneous rock, anorthosite cools and
crystallizes at depth. This is a slow process allowing larger crystals to form.
The mineral grains are easily visible in a hand specimen. The predominant
mineral is a calcium-rich plagioclase. Colors vary from white to very dark gray
and even dark green (and even iridescent blue in the case of a variety of
plagioclase called Labradorite). Weathered
surfaces are often tan.
Peridotite
Another intrusive rock that is relatively rare at the
surface of the Earth but probably makes up the bulk of the Earth’s mantle—from
about 40 miles below your feet all the way down to the iron-nickel core. The
defining minerals making up peridotite are olivine and pyroxene. The olivine
shows up a green glassy crystal grains (larger crystals of olivine of gem
quality are cut and polished and referred to as peridot in the gem and jewelry trade). The pyroxenes are a semi-glassy
or satin brown, dark brown or black.
Hence, peridotites appear crystalline, dark and
greenish.
