Features of ALH84001
The meteorite ALH84001 is a chunk of igneous rock, very similar to terrestrial rocks which are formed by solidification of volcanic lava. Most of the minerals of which it is made are silicates. It is crisscrossed by a large number of fault lines. You see its interesting features when you break it along a fault.
The largest unusual features of ALH84001 are a large number of small globules of carbonate minerals, ranging from 1 micron (0.001 mm) to 250 microns (1/4 mm) across. These globules sit in pores and along fractures in the rock, and (as the picture shows) some of them are themselves fractured -- indicating that ALH84001 suffered a major shock some time after the carbonates were formed.
ALH84001 contains organic materials called "polcyclic aromatic hydrocarbons," or PAH's. These compounds are known to occur on other meteors and on interplanetary and interstellar dust particles. They also occur on Earth, both as by-products of industrial processes and as products of decomposition of ancient life. There are hundreds of thousands of compounds which are classified as PAH's, but only a couple hundred or so are found in ALH84001. The concentration of PAH's is largest where the carbonate globules are, except that the concentration is very low near the outer skin of the meteorite -- the part that would have melted when it fell through the atmosphere 13,000 years ago. The concentration of PAH's starts to rise when you get about half a millimeter into the interior of the rock.
The carbonate globules have two interesting aspects to them. First, they have black and white bands around their outer rims, in contrast with the orange appearance of the globule itself. Second, the surfaces away from the rims have several odd features that reveal themselves under electron microscopy.
The rim of a typical, largish (about 50 microns across) globule actually consists of alternating iron-rich and magnesium-rich bands. The iron-rich bands are a mixture of mostly magnetite crystals, each 0.01 to 0.1 micron in size, with a little bit of an iron-sulfur compound called pyrrhotite, whose crystals are about the same size as the magnetite crystals but collected in domains which stretch 5-10 microns along the rim.
The surfaces of the carbonate globules, far from the rims, are particularly interesting. There are regions where the large carbonate crystals are cut by porous regions where the carbonate is fine-grained and seems to have been partially dissolved. Interspersed with the fine-grained carbonate are many tiny but elongated particles (about 0.1 micron long) of another iron-sulfur mineral called greigite.
Under high resolution scanning electron microscopy, some very interesting features on the surfaces of the carbonate globules become evident. Near the rims, where there is a large concentration of magnetite and pyrrhotite, there is also a large number of ovoid objects and irregularly shaped, tubular objects. These objects are roughly the same size and shape as the magnetite and pyrrhotite particles around them.
These curious ovoid and tubular objects also occur near the centers of the carbonate globules, away from the rims. The ovoids are up to about 0.1 micron long, the tubes up to perhaps 0.05 micron.