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, many industrial slags have vesicles. Vesicles and metal together in the same “rock” are a good field mark for slag.
Vesicles in Terrestrial Rocks
Basalts and related volcanic rocks (andesites, dacites) form when volcanic lava or magma cools. Not all basalts are vesicular, but vesicular basalts are very common on Earth.
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. Some lunar meteorites contain amygdules, however, that formed after the meteorite landed on Earth (see Shişr 166, below).
Vesicles in Basalts from the Moon
Vesicles in Basaltic Meteorites
Vesicles only develop in rocks that cool from a liquid – 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 and diogenites which come from a large asteroid like 4 Vesta that had volcanoes, martian meteorites (Mars has some really big volcanoes), and about 5% of the lunar meteorites. So far, there have been no vesicular martian or lunar basaltic meteorites discovered. (Caveat: I’m aware of one vesicle in a martian basalt (EETA 79001). A few eucrites and diogenites are moderately vesicular, however.
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 origin.
Some lunar meteorites have vesicles that formed by impact of asteroidal meteorites on the Moon. A spectacular example is Shişr 166 (the only lunar meteorite to have been found at night!).
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 spherical.
Keep in mind, however, that meteorites are very rare and lunar meteorites are exceedingly rare – only about 1 in 1000 meteorites are from the Moon.
Vesicular Meteorite Fusion Crusts
Many stony meteorites have vesicular fusion crusts. As the surface melts when the meteorite passes through the atmosphere, gases in the meteorite are released. Some of that gas becomes trapped in the glassy melt as the melt cools.
Many lunar meteorites are regolith breccias, and some of these have fusion crusts that are thick (up to 2 mm) 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 regolith (“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.
Tiny lunar meteorite QUE 94281. It also has a highly vesicular fusion crust because it is a regolith breccia. Centimeter cube for scale. Photo credit: NASA/JSC
Many people have contacted me saying, “My rock looks just like QUE 94281.” Below, on the right, is a photo that one such person sent me. Superficially, the rocks do resemble QUE 94281, on the left, but not in detail. QUE 94281 is a fragment broken from a larger stone, so it has some rough edges, like the basalts on the right. The fusion crust on QUE 94281 coats only part of the stone. As in PCA 02007 above, the interior of QUE 94281 does not contain vesicles because the interior was never molten. The rocks on the right 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 the meteorites. Basaltic rocks like those in this photo are used for landscaping and in barbecue grills.
If you have a vesicular rock, then it is not a meteorite. Such rocks are very common on Earth but are exceedingly rare among meteorites.