|North America||Bisbee, Cochise County, Arizona||4 ½" x 3 ½" x 2"|
To the right is a drawing of the crystal structure (that is, the arrangement of the atoms) of malachite. The red spheres represent oxygen, the brown ones OH, blue spheres copper, and the purplish triangles represent co.i groups with C at their centers. Azurite is Cu3(CO3)2(OH)2. The black box is the unit cell.
Now, don't expect that you will glean a lot from looking at this picture, but perhaps it will raise a question in your mind: How does anyone figure out how atoms (which we cannot see) in a mineral are arranged?
Crystal structures are most often determined by a technique called X-ray diffraction. A small fragment of the mineral is inserted into a narrow beam of X-rays. Unlike medical X-ray examinations, which work because hard tissues absorb X-rays, X-ray diffraction works by "reflecting" X-rays from planes of atoms in the crystal and measuring their intensities. (It is really diffraction, but reflection is an easier concept to think about, and operationally, the two phenomena look about the same.) By making hundreds to thousands of such measurements on a single crystal, and doing some number crunching, the arrangement of atoms (the crystal structure) can be determined .