Dioptase on Plancheite (#41)
|Africa||Djoue River, ner Pimbi, Republic of Congo||3 ¾"x 2 ¼" x ¾"|
The dark green crystals are dioptase, CuSiO2(OH)2, and the light blue mineral is plancheite, Cu8Si8O22(OH)4 • (H2O). which also occurs in specimen #36. The top picture is the unit cell of dioptase, and the bottom one is a portion of the crystal structure of plancheite, with some atoms removed for clarity.
These two minerals illustrate the remarkable variety of silicate structures - those based on Si and 0. Dioptase is a ring silicate (or cyclosilicate). You can see two rings in the unit cell. The blue-green Cu atoms connect adjacent rings. The structure of plancheite, on the other hand, is based on chains of SiO4 units. From the perspective of the drawing, the chains project back into the screen and slightly to the right. They are connected in groups of four parallel chains each and, again, connected by the blue-green spheres representing Cu atoms.