METEORITE OR METEORWRONG?
vesicles & amygdules
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 vesicles because
most meteorites were 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.
||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 never occur in meteorite fusion crusts. Click
on image for enlargement.
||This is meteorwrong number 018.
It's 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
||A highly vesicular basalt from
Hawaii. Highly vesicular volcanic rock is also known as pumice.
||Vesicular rocks occur all
over the world. This one is from Australia.
(Photo by Max McCosker)
Basalts are the rocks that form
when volcanic lava or magma cools. Not all basalts are vesicular, but
vesicular basalts are very common on Earth.
See meteorwrongs 016
| 018 | 038
| 044 | 068
| 079 | 112
| 137 | 141
| 161 | 208
of the vesicular samples that people bring to us are industrial slags.
Slags are often vesicular.
OK, some rare meteorites have vesicles
Most meteorites come from asteroids, and almost all asteroids
are too small to have volcanoes, thus very few meteorites are igneous
rocks. The igneous rocks among meteorites are the eucrites and diogenites
(believed to come from a large asteroid like Vesta
that had a volcano), most of the martian meteorites (Mars has some really
big volcanoes), and a few 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
(width: probably ~10 cm) and on the right is a slice of the diogenite
700. These two meteorites are the most vesicular meteorites of
which the author is aware. Among all known meteorites, only 1.36%
are eucites and 0.54% are diogenites. Most eucrites and diogenites
are not vesicular. Photo courtesy of Ray Stanford. Click on image
| 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 (see
meteorwrong no. 068 for more information).
It is possible that some day someone will find a vesicular lunar
When two asteroids collide, melting may occur and gas may be
released. Sometimes the impact melt may trap
gas bubbles when it cools. This is a special kind of igneous meteorite,
one even rarer than meteorites of volcanic origin.
Some chondrites have vesicular fusion crusts (for example, see Cynthiana).
Lunar meteorites are unusual in that many of those that are regolith
breccias have fusion
crusts that are highly vesicular. The best examples are QUE
03069 and PCA 02007,
although the effect can be also seen in ALHA
81005 and Calcalong
Backscattered-electon 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. Longest dimension:
1.3 cm. The vesicles are all smaller than 1 mm in diameter. Image
courtesy of Ryan Zeigler.
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 bee 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
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.
A large impact can shock-compress
lunar regolith into a coherent rock, a regolith breccia. If that
rock is blasted off the Moon by an impact (possibly the same
impact that formed the breccia), the rock becomes a meteoroid.
When the meteoroid passes through the Earth's atmosphere, it
became a meteor. When the exterior of the meteor 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
The important distinction is that in lunar meteorites the interior
usually does not have vesicles but the exterior does.
In many terrestrial rocks, like the basalts above and industrial slags,
the opposite is observed - the interior is vesicular and the exterior
skin has fewer vesicles.
Among lunar meteorites, there is one exception to the "no
internal vesicles" rule. The four paired stones of Dhofar
081/280/910/1224 have some vesicles in the 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 spherical.
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.
line: If you have a vesicular rock, even one with a glassy
coating, it's probably not a meteorite. Such rocks are very common
on Earth but are exceedingly rare among meteorites.
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
or amygdular. 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).
|This is meteorwrong
number 016, an amygdaloidal basalt. Click
on image for enlargement. Meteorwrong 185 is
another good example.