Hematite after Magnetite (#43)
|South America||Patagonia, Argentina||3 ½" x 2" x 1 ¾"|
Magnetite is Fe3O4 and has a unit cell that is cubic in shape. This specimen looks for all the world like magnetite. It has crystals of the typical octahedral shape (two square pyramids base to base), and it is black. Magnetite has certain properties that make it easy to identify. First, it is the only mineral that is strongly magnetic, so bringing a magnet close to it is a sure test. It is also quite dense, over 5 g/cm3, which means that it feels about twice as heavy as a piece of "average rock" the same size. It has a black streak. Streak is the color of the powder produced when the mineral is rubbed on a piece of unglazed porcelain called a streak plate.
If you pick this specimen up, you will consider it pretty heavy for its size. But if you touch a magnet to it, nothing will happen. If you rub it on a streak plate, the streak will be bright to dark red-brown. You will think it is not magnetite, and you will be correct.
As specimen #37 is a pseudomorph, this specimen is a pseudomorph of hematite after magnetite. All of the physical properties associated with hematite are in evidence except the shape of the crystals. Hematite crystallizes in a different crystal system than does magnetite, and its unit cell is not a cube, but a prism with a "diamondshaped" cross-section, so hematite crystals look very different from this. The specimen formed as magnetite (from a vapor or gas, according to the label) to begin with, but thereafter it oxidized to hematite, tiny volume by tiny volume, leaving the crystal shape intact.