Meteorite or Meteorwrong
Vesicles in Meteorites
The Dictionary of Geologic
Terms (R. Bates & J. Jackson, eds) defines vesicle as "a small
cavity in an aphanitic or glassy igneous rock, formed by expansion of a
bubble of gas or steam during solidification of the rock." Such a rock
is said to be vesicular. Only igneous rocks - rocks that cooled from a molten
magma - can have vesicles. Very few (less than 1 in 1000) meteorites have
interior vesicles because the interior of most meteorites was never molten.
Many terrestrial rocks have vesicles, however. Also, almost every sample of
industrial slag that we've seen has vesicles. Vesicles and metal together in
the same "rock" are a good field mark for slag.
Terrestrial Rocks with Vesicles
are the rocks that form when volcanic lava or magma cools. Not all basalts
are vesicular, but vesicular basalts are very common on Earth.
Pictured here is a
vesicular basalt from Hawaii. The field of
view is about 6 cm.
The surface at the top of the photo is where the molten lava was
exposed to air. The exposed portion cooled quickly, leaving a glassy,
shiny surface. The surface somewhat resembles a meteorite fusion crust. Meteorite fusion crusts
are usually smoother than this, however. Also, you can see the circular
shapes of broken gas bubbles in the crust of this rock; such features are
very rare in meteorite fusion crusts. Click on image for enlargement.
This is also a vesicular basalt.
As on the photo above, the crust on top is where the molten lava was
exposed to air. Basalts come in a variety of colors, mostly gray or black
to rust colored.
A highly vesicular basalt
from Hawaii. Highly vesicular volcanic rock is also known as scoria, or
pumice if the density is very low.
Vesicular rocks occur all
over the world. This one is from Australia. (Photo credit: Max McCosker)
An amygdule is "a gas
cavity or vesicle in an igneous rock which is filled with such secondary
minerals as zeolites, calcite, quartz, or chalcedony." Such a rock
is said to be amygdaloidal. Amygdules form when
fluids containing dissolved minerals flow through the rocks and deposit
the minerals as solids in the vesicles. Lunar basalts are not
amygdaloidal because the Moon is so dry that there are no fluids (and,
apparently, there never were).
Vesicles in Basaltic Meteorites
Vesicles only develop in rocks
that cool from a liquid Ė an igneous rock. Most meteorites come from
asteroids, and almost all asteroids are too small to have volcanoes, thus
very few meteorites are igneous rocks. Most such rocks among the meteorites
are basalts. Most common are the eucrites
(believed to come from a large asteroid like Vesta that had volcanoes), most
of the martian meteorites (Mars has
some really big volcanoes), and some lunar
meteorites. So far, there have been no vesicular martian or lunar
basaltic meteorites discovered.
A few eucrites and diogenites
are moderately vesicular, however.
On the left is a slice of the eucrite Ibitira (width: probably ~10 cm) and
on the right is a slice of the diogenite Dhofar 700. These two meteorites are the
most vesicular meteorites of which the author is aware. Among all known
meteorites, only 1.36% are eucrites and 0.54% are diogenites. Most eucrites
and diogenites are not vesicular. Photo courtesy of Ray Stanford. Click on
image for enlargement.
Two vesicular lunar basalts, sample 15556 from
the Apollo 15 mission (left, cube is 1 inch) and sample 71155 from the
Apollo 17 mission (cube is 1 cm). Note that the vesicles are round, not elongated.
This occurs because lunar basaltic magmas had very low viscosity and lunar
gravity is low. It is possible that someday someone will find a vesicular
basaltic lunar meteorite. Click on image for enlargement.
Vesicles in Meteoritic Impact Melts
When two asteroids collide,
melting may occur and gas may be released. Sometimes the impact melt traps gas bubbles when it cools. This is a
special kind of igneous meteorite, one even rarer than meteorites of volcanic
A few lunar meteorites have
vesicles that formed by impact of asteroidal meteorites on the Moon. A
spectacular example is Shişr 166.
This is a sawn slice of lunar meteorite Shişr 166. The gray portions are veins
of solidified impact melt. Several vesicles occur in the impact melt. A few
of the vesicles are filled with calcite (the whitest material) that
precipitated from aqueous fluids after the meteorite landed in Oman. Such
features are called amygdules (above).
Another vesicular lunar
meteorite are the four paired stones of Dhofar 081/280/910/1224. These stones have vesicles in
the glassy matrix because the matrix was once molten and probably consisted
of melted regolith that contained solar wind gases. It's probably more
accurate to call these cavities vugs, not vesicles, because most are not
Slices of lunar meteorite Dhofar 910, each about 1 cm across. Notice
that the clasts do not have vesicles or vugs but
the once-glassy (now devitrified) matrix does. Again, the voids are all
less than a millimeter in size. Photos courtesy of Haberer-Meteorites.
Keep in mind, however, that
meteorites are very rare and lunar meteorites are exceedingly rare - less
than 1 in 1000 meteorites are from the Moon.
Vesicular Meteorite Fusion Crusts
Many stony meteorites have
vesicular fusion crusts (for example, see Cynthiana).
As the surface melts when the meteorite passes through the atmosphere, gases
in the meteorite is are released. Some of that gas becomes trapped in the
glassy melt when the melt cools.
Many lunar meteorites are regolith breccias, and some of these have fusion
crusts that are thick and highly vesicular. The best examples are QUE 03069 and PCA 02007, although the effect can be
also seen in ALHA 81005 and Calcalong Creek. At one time, all of the
material of a regolith breccia was fine grained "soil" on the
surface of the Moon. Soil grains exposed at the very surface of the Moon
absorbed ions emitted by the sun as solar wind. Most of the ions were of
gaseous elements like hydrogen, helium, and nitrogen. Impacts of small
meteoroids on the Moon mixed and stirred the upper part of the regolith. In a
location where there has not been a recent large impact, nearly all the
grains in the upper few meters of the regolith will contain solar-wind
implanted gases because over millions of years all grains spend some time at
the surface. On Earth, the solar wind is absorbed by the atmosphere, so there
are no Earth rocks with solar-wind implanted ions. Some meteorites from the
asteroid belt have solar-wind gases, but none have the high levels found in
lunar meteorites because the Moon is closer to the sun. When the exterior of
the meteoroid is heated by the friction of the atmosphere, it melts and the
gases are released, forming gas bubbles that get trapped in the glass when
the glass cools.
image of lunar meteorite PCA 02007, a regolith breccia. At the top and right is
the glassy, vesicular fusion crust that occurs on the outside of the
meteorite in this photo; the vesicles are black in
the image. At the bottom is the brecciated interior of the meteorite. It
was never molten, so there are no vesicles. Longest dimension: 1.3 cm.
The vesicles are all smaller than 1 mm in diameter. (Image credit: Ryan
This is a photo of tiny lunar meteorite QUE 94281. It has a highly vesicular
fusion crust because it is a regolith breccia.
Many people have contacted me saying, "My rock looks just like
QUE 94281." Above is a photo that one such person sent me. Superficially,
the rocks do resemble QUE 94281, but not in detail. QUE 94281 is a fragment
broken from a larger stone, so it has some rough edges, like the rocks
above. The fusion crust on QUE 94281 coats only part of the stone. As in PCA 02007 above, the interior of QUE94281 does not contain
vesicles (although that's hard to see in this photo) because the interior
was never molten. The rocks above are not regolith breccias, but
terrestrial basalts, often called scoria. Notice in the upper right
stone that there are fewer vesicles in the chilled top
than in the interior - the opposite of what is seen in meteorites. Rocks like
those in this photo are used for landscaping and in barbeque grills.
If you have a vesicular rock, then it's not a meteorite. Such rocks are
very common on Earth but are exceedingly rare among meteorites.