Pyromorphite (#74)

An aesthetic cluster of lustrous hexagonal barreloids of yellow-green color to 7/16", many doubly terminated.
North America Bunker Hill, Kellogg, Idaho 1 ¾" x 1 ½" x 1"

 

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Pyromorphite has the chemical composition Pb5(PO4)3CI. It ranges in color from yellow to green to orangish or brown. Pure pyromorphite is colorless, so the various colors are due to impurities.

Another mineral, known as apatite, has the chemical formula Ca5(PO4)3(OH,Cl,F). The crystal structures of the two are identical. Notice that the formulas of these two also look very similar. Pyromorphite has Cl at the end of its formula, whereas apatite may have hydroxyl (OH), chlorine, or fluorine (or some combination thereof). Other than that, only lead (Pb) or calcium (Ca) distinguish these two, chemically. Pyromorphite is softer, with hardness 3.5-4 on the Mohs scale of hardness, whereas apatite is the defining mineral for hardness 5.

Why belabor these differences and similarities? Because your bones and teeth are made largely of hydroxylapalite, Ca5(PO4)3(OH). Suppose your teeth were pyromorphite. Since pure pyromorphite is white, your teeth probably wouldn't be the color of this specimen. But at hardness 3.5-4, you would visit the dentist a lot more frequently. And, of course, getting lead poisoning from your own teeth would really be annoying. Fortunately, none of that occurs. Exchanging one element for another in the chemical composition of a mineral can make big changes in that mineral's physical properties.