vesicles & amygdules

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 to rust.
	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 |209

Many 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 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 eucites 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 (see meteorwrong no. 068 for more information). It is possible that some day someone will find a vesicular lunar meteorite.

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 Creek.

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 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.

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 cools.

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.



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.

Bottom 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.

Prepared by:

Randy L. Korotev

Department of Earth and Planetary Sciences
Washington University in St. Louis

Please don't contact me about the meteorite you think you’ve found until you read this and this.

e-mail: korotev@wustl.edu