Sphalerite with Pyrite and Galena (#90)
|North America||Park City, Wasatch County, Utah||1 ¼" x 1 ¼" x 1"|
Sphalerite (ZnS), pyrite (FeS2), and galena (PbS) have at least one thing in common: they are all metal sulfides. Many ore minerals (that is, minerals that contain elements of such value that they are mined as "ores" to obtain those elements) are sulfides. The desired elements may be found in other minerals, but perhaps those minerals are harder to process or are not found in sufficient concentrations to make a mining venture profitable. Sphalerite is mined for zinc, and galena is mined for lead. Although you might think that pyrite should be mined for iron, the "Learn More" for specimen #73 explains why we don't mine pyrite for iron.
The similarity in the formulas of sphalerite and galena might cause you to think their atoms should be arranged in much the same way. Not so. Because Zn atoms are relatively much smaller than S atoms, 4 S atoms can be fit around one Zn atom, resulting in the structure to the right. (If all these atoms were replaced by carbon atoms, we would have diamond.)
The Pb atoms in galena are comparatively larger than Zn atoms, more closely approaching the size of the S atoms. As a result, 6 S atoms will fit around one Pb atom (and vice versa), so that galena has the same structure as halite - common table salt. The relative sizes of atoms is a major governing factor in the crystal structures Mother Nature adopts.