LUNAITES
About 244 rocks that have fallen to Earth as
meteorites are demonstrably from the Moon.
Numerous rock samples were brought back by the American Apollo missions of the
latest 1960s and early 1970s, but those samples are inaccessible except to a
few planetary geology researchers.
Lunaite
(lunar brecciated anorthosite; lunar impact melt anorthosite breccia) (6.5 mm
across) - this is a sample from a much-celebrated Moon rock that was found in
Africa's Sahara Desert. The fragment is small, but it is sufficient to show
the overall nature & texture of the rock.
This is the NWA 482 Meteorite (Northwest
Africa 482 Meteorite), found in Algeria in the year 2000. The
lithology is a brecciated anorthosite. A few clasts of other
lithologies have been identified as well, so a better term is a polymict
breccia. This rock formed during an ancient impact event early in the
Moon's history. The full rock term is often given as polymict impact melt
breccia. The photo shows its brecciated & fractured nature. The
breccia consists of light-colored clasts in a very dark matrix.
The light colored clasts are ferroan
anorthosite. Anorthosite is an intrusive igneous rock that is exclusively
or near-exclusively composed of plagioclase feldspar. There are several
different plagioclase feldspar minerals. This rock is 80-90% composed of anorthite,
a very calcium-rich plagioclase.
The dark-colored, fine-grained matrix of this rock is
reported to contain anorthite plagioclase feldspar, olivine, pyroxene
(pigeonite & augite), troilite (FeS), metallic iron-nickel alloy, ilmenite,
and whitlockite.
Available information indicates it is likely derived
from the lunar highlands on the far side of the Moon (the side always facing
away from Earth). The anorthosite portion of this rock represents the
earliest lunar crust (4.5 billion years old). This date indicates the
initial formation of our solar system. The oldest Martian rocks are 4.5
billion years. The oldest rocks from the asteroid belt are 4.5 billion
years. The oldest known Earth materials are 4.4 to 4.5 billion years.
Isotopic analysis of NWA 482 has provided dates for
other significant events in this rock's history. The major brecciation
and impact melt event occurred at 3.75 billion years ago. Another
significant thermal event occurred at 2.4 billion years ago.
Available dates that indicate when this rock was
ejected from the Moon's surface vary - for example, at ~1 million years ago and
at ~280,000 years ago. Available dates indicating arrival time on Earth
also vary, including 60,000-120,000 years ago and 8600 years ago - the younger
date is more likely, considering the relatively unweathered appearance of the
original rock.
The above info. is mostly synthesized from published
abstracts during Geological Society of America and Lunar & Planetary
Science meetings held from 2001 to 2006.
Lunaite
(lunar mare basalt; 253 grams) - this rock was collected by American astronauts
on the Moon itself & brought back to Earth. This is a lunar mare
basalt. The Moon's surface has several huge, dark-colored,
subcircular to irregularly-shaped depressions called maria (see
pic of the Moon). The lunar maria are floored by thick successions of
basalt lava flows. This sample is a 3.3 billion year old grayish basalt
from one of these maria. It has some vesicles - the small rounded
cavities representing trapped gas bubbles when the lava initially
solidified. Cosmic radiation exposure dating indicates that this was
exhumed from the lunar subsurface 228 million years ago (during the Triassic),
presumably by an impact event.
CMNH public display, Cleveland Museum of Natural
History, Cleveland, Ohio, USA.
Surface collected on the Moon by Alan Bean on 20
November 1969.
A very large, unique Moon rock was found in
2007 in Morocco. See photos &
info.