Vesicles and 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 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 very rare in meteorite fusion crusts.

Vesicular basalt from New Mexico. 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. Photo credits Randy Korotev

Left: Vesicular basalt with olivine (olive green) phenocrysts. Right: Near-ocean vesicular basalt with whelk shells. Photo credits: Randy Korotev

Vesicles are not always circular. 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 lava flows and stretches the gas bubbles (sawn face). Photo credit: Randy Korotev.

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). Photo credit: Randy Korotev.

These photos of vesicular and amygdaloidal rocks were all sent to me by persons who thought that they might be meteorites. None of them are meteorites.

Vesicles in Basalts from the Moon

About 5% (I have not actually counted) of the basalt samples collected on the Apollo missions to the Moon are vesicular. At the top is 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) and . 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, but so far it hasn’t happened. Photo credits: NASA.

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.

Two samples of the eucrite Ibitira. This one of the most vesicular meteorites of which the author is aware. Among all known stony meteorites, only 2.3% are eucrites. Most eucrites are not vesicular. Photo credits: top, Meteorites Australia; bottom, American Museum of Natural History.

Diogenite Dhofar 700 is another highly vesicular volcanic meteorite. Among all known stony meteorites, only 0.54% are diogenites. Most diogenites are not vesicular. Photo credit: Ray Stanford.

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.

A few 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 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). 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’s probably 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. Photo credit: Randy Korotev.

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 is 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 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.

Two sides of lunar meteorite QUE (Queen Alexandra Range) 93069. The fusion crust (bottom) is strikingly vesicular because the meteorites 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. The cube is 1 cm square. Photo credits: NASA.

Two sides of lunar meteorite PCA (Pecora Escarrpment) 02007. This meteorite is a regolith breccia with a vesicular fusion crust. The cube is 1 cm square. Photo credits: NASA.

Backscattered-electron image of 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.

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 QUE 94281 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 barbecue grills.

Bottom line:

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.