Rocks are often made of small mineral grains that can't be seen clearly without a microscope. To see these small grains, scientists grind and polish rock samples very thin (0.03 millimeters) so light can pass through them. This picture is a microscopic view, about 2.3 millimeters across, of a martian meteorite similar to that seen in slide #35. The brown areas are grains of the mineral pyroxene and the clear white areas are the mineral plagioclase. These are the two most abundant minerals in basalt, both on Earth and Mars. The black areas are magnetite, an iron-oxide mineral.
Photograph by Allan Treiman, Lunar and Planetary Institute