Vesicles, vugs, and amygdules

If you have a vesicular rock that does not have a fusion crust, then it is not a meteorite.

From the Self-Test flowchart

The Dictionary of Geologic Terms (1984, 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 – or impact melts 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.

Vesicular basalt. 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 rare in meteorite fusion crusts. Photo credit: Randy Korotev
Vesicular basalt from New Mexico. As on the photo above, the glassy crust on top is where the molten lava was exposed to air and cooled quickly. It is not a meteorite fusion crust. A meteorite fusion crust would not be this flat. Basalts come in a variety of colors, mostly gray or black to rust colored. Photo credit Randy Korotev
Left: Vesicular basalt with olivine (olive green) phenocrysts. Right: Near-ocean vesicular basalt with whelk shells in some vesicles. Photo credits: Randy Korotev
Vesicles that are not circular are called vugs. Photo credit: Randy Korotev.
Highly vesicular volcanic rock is also known as scoria or pumice if the density is very low. Photo credit: Randy Korotev.
Elongated vesicles form when the lava flows and stretches the gas bubbles before the melt solidifies (sawn face). Photo credit: Randy Korotev.

Amygdules and amygdaloidal basalts

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 a rock 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, however, contain amygdules that formed after the meteorite landed on Earth (see Shişr 166, below).

None of these rocks has a fusion crust. Chondrules in chondritic meteorites are not this light colored.
None of these rocks has a fusion crust.

Vesicles in basalts from the Moon

About 5% (I have not actually counted) of the large basalt samples collected on the Apollo missions to the Moon are vesicular. Someday a highly vesicular, basaltic lunar meteorite may be found but so far that has not happened. Vesicular basalts may not survive the shock of being blasted off the Moon and passing though Earth’s atmosphere

At the top is basalt sample 71155 from the Apollo 17 mission (cube is 1 cm) and at the bottom is sample 15556 from the Apollo 15 mission (cube is 2 cm). Note that the vesicles are round, not elongated, because lunar basaltic magmas had very low viscosity (it is “runny”) and lunar gravity is low. These rocks do not have fusion crusts because they are not meteorites. Photo credits: NASA.
A slice of meteorite NWA 14137, a lunar basalt. NWA 14137 is the most vesicular basaltic lunar meteorite of which I am aware. Because of the low lighting angle (from the right), the vesicles stand out and look like small craters because all have shadows are on the right-hand side. Photo credits: Scott McGregor

Vesicles in other 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 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 meteorites discovered. (Caveat: I am aware of one vesicle in a martian basalt (EETA 79001). A few eucrites and diogenites are moderately vesicular, however.

Two samples of the eucrite Ibitira. This is one of the most vesicular meteorites of which the author is aware. Among all known stony meteorites, only 2.4% are eucrites. Most eucrites are not vesicular. Photo credits: top, Meteorites Australia; bottom, Jon Taylor
Diogenite Dhofar 700 is another vesicular volcanic meteorite. Among all known stony meteorites, only 0.8% are diogenites. Most diogenites are not vesicular. Photo credit: Ray Stanford
A slice of volcanic angrite Oued Namous 001 from Algeria. Meteoritical Bulletin description: “…scattered gemmy green olivine phenocrysts up to 8 mm in diameter. Spherical vugs are ubiquitous.” Only rocks that were once molten contain vesicles and vugs. Angrites are rare meteorites, only about 0.06% of stony meteorites are angrites. Photo credit: Scott McGregor 

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!).

A sawn slice of lunar meteorite Shişr 166. The gray portions are veins of glassy, solidified impact melt. The vesicles occur only in the gray impact melt, not in the clasts. A few of the vesicles are filled with calcite (whiteish) that precipitated from aqueous fluids after the meteorite landed in Oman. Such features are called amygdules (above). Millimeter ticks at bottom. Photo credit: Randy Korotev.

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 is more accurate to call these cavities vugs, not vesicles, because most are not spherical.

Slices of lunar meteorite Dhofar 081 (top) and Dhofar (280) bottom. The voids are all less than a millimeter in size. Again, they occur in the glassy, solidified melt, not in the clasts. Photo credit: Randy Korotev.

Keep in mind, however, that meteorites are very rare and lunar meteorites are exceedingly rare – only about 7 in 1000 meteorites are from the Moon.

Some other meteorites with vesicles or vugs

Vesicles in meteorite fusion crusts

Some 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 Queen Alexander Range (QUE) 93069 and Pecora Escarpment (PCA) 02007, although the effect can be also seen in Allan Hills (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 spent 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 are regolith breccias containing solar-wind gases but none have levels as high as lunar meteorites because the moon is closer to the sun than is the asteroid belt.

Two sides of lunar meteorite QUE (Queen Alexandra Range) 93069. The fusion crust (bottom) is strikingly vesicular because the meteorite is a regolith breccia. Note that here are no vesicles in the interior of the meteorite (top) because the interior did not get hot enough to melt. Centimeter cube for scale. Photo credits: NASA.
Two sides of lunar meteorite PCA (Pecora Escarpment) 02007. This meteorite is a regolith breccia with a vesicular fusion crust. The cube is 1 cm square. Photo credits: NASA.
Backscattered-electron (BSE) image of lunar meteorite PCA 02007. At the top and right is the glassy, vesicular fusion crust that occurs on the outside of the meteorite in the photos above; the vesicles are black in the image. Note the sharp contact with the unmelted interior. Image credit: Ryan Zeigler
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 any 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.

Left: Two views on lunar meteorite QUE 94281. Right: Fragments of terrestrial basalt.

Vesicular fusion crusts in other types of meteorites:

Bottom line:

If you have a vesicular rock with no fusion crust, then it is not a meteorite. Such rocks are very common on Earth but are exceedingly rare among meteorites.