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 interior vesicles because the interor of most meteorites was
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
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. Click on image for enlargement.
||This is 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)
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).
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.
If you have a vesicular rock it's not a meteorite. Such rocks are
very common on Earth but are exceedingly rare among meteorites.
vesicles in meteorites
vesicles in meteorite