21. ALH 84001 in Thin Section

This slide shows a microscope view, about 3 millimeters across, through a thin slice of ALH 84001. Because so many of the grains and structures in rocks are small, geologists usually need to examine them through a microscope. Rocks must be sanded and polished to very thin slices, usually 0.03 millimeters thick, to see through them well.

The ALH 84001 meteorite, host to the possible microfossils and traces of past life, is an igneous rock. This means it formed originally from molten lava, which cooled and crystallized to solid rock. ALH 84001 solidified from basalt lava, the same kind that erupts in Hawaii, along the Earth's mid-ocean ridges, and once erupted in western Washington state (see slide #29). Most of ALH 84001 is grains of the igneous mineral pyroxene, which looks white in this view. Boundaries between grains are usually visible as lines of darker fractures and other minerals. The black mineral grains in this view are the igneous mineral chromite.

Allan Treiman (Lunar and Planetary Institute)

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