Chrysocolla with Quartz (#33)
|North America||Bagdad, Gila County, Arizona, USA||7" x 5 ½" x 4"|
Remember the definition of "mineral" in Learn More for the introductory screen? It required, among other things, that a mineral have a definite (not necessarily fixed) composition and a fixed arrangement of atoms (i.e., a crystal structure). Are there exceptions to these rules? Well, perhaps. There are a few minerals that, over the years, have consistently been described as "amorphous," literally meaning "without form." Such minerals have no regular and fixed arrangement of atoms, but they have been considered minerals for so long that most mineralogists are content to simply call them amorphous minerals - an oxymoron, perhaps, but a useful one. See Learn More for Specimen #82 for another amorphous mineral.
One of the most important methods for investigating the crystal structures of minerals (and synthetic crystalline compounds, as well} is X-ray diffraction. This is not the place to expound on X-ray diffraction, but for crystalline materials it yields data that allow the determination of such things as the unit cell dimensions and arrangement of atoms. Xray diffraction patterns of chrysocolla are essentially featureless, indicating either that there is no regular arrangement of atoms, or that the crystals are so incredibly tiny that the crystal structure is incapable of yielding coherent diffraction results.
Don't be fooled by the absence of visible crystals, though. Many minerals with very clearly defined crystal structures sometimes adopt a rounded or massive habit with no visible crystals, so that trait does not make a mineral amorphous. A recent study suggests that chrysocolla is a mixture of two micro-crystalline minerals - perhaps, but the jury (i.e., the scientific community) has not returned a verdict on that. Just remember: Every rule has exceptions, and "amorphous minerals" are exceptions.