Aquamarine with Albite and Muscovite (#27)
|Asia||Haramosh, Skardu Road, Gilgit Division, Pakistan||2 ¼" x 2 ¼" x 1 ¼"|
The dominant shape here is a long, 6-sided prism. Although the reason for the specific crystal faces on a crystal is often unclear (see Learn More, specimen #26}, the overall symmetry can be traced to the shape of the unit cell. In Learn More under specimen #17, the unit cell was described as an imaginary, tiny box that contains the entire formula (or integral multiples thereof) and symmetry necessary to build the entire specimen by stacking unit cells together. To the right is the unit cell of beryl (of which aquamarine is a variety) looking down the long dimension of the crystal. The details are obscured by the small size (you can see the red spheres - they represent oxygen atoms). The unit cell is outlined in black, and it has the "hexagonal" shape (a=b≠c, α=β=90°, y=120°). Let's look at the unit cell outline, and stack three together, below.
Now the hexagonal arrangement of unit cells is seen to be mirrored in the shape of the physical crystal. Think of it as assembling tiny Legos. With Legos so small, you could stagger them to make the small, tilted faces on the top edges of this crystal, and the surface would still look smooth and planar. You just have to remember that the unit cells are only a few Angstroms across, which is smaller than the wavelengths of visible light, so the surfaces that are jagged at the A level look perfectly smooth to your eye.