|South America||Minas Gerais, Brazil||5 ¾" x 2" x 1 ½"|
We discussed cleavage, the way in which a mineral breaks along planar surfaces, in the "Learn More" for specimen #6. Those minerals that show cleavage break along directions in the crystal structure where there are relatively few bonds or weaker bonds than in the rest of the structure. Topaz provides a good example. The chemical formula is A12Si0 .(0H,F)2, where (OH,F) means that the hydroxyl ion and fluorine may substitute for each other. The picture below is the structure of topaz, with small blue spheres representing silicon atoms, light blue spheres Al, red 0, and green OH and F. The red line (a plane on edge) is the cleavage plane. Note that it breaks no Si—0 bonds (which are strong), and more Al-(OH,F) (weaker) than Al—0 (stronger) bonds.
You might think that good cleavage would make a mineral soft, but it is not necessarily so. We introduced the Mohs hardness scale with specimen #7. On the scale of 1-1 0, topaz is B. The weakest bonds are aligned along certain planes, which leads to good cleavage, but none of the bonds are very weak - some are just "weaker" than the others. The compact, 3-dimensional network of relatively strong bonds makes topaz harder even than quartz.
Naturally blue topaz is rare and valuable. The blue color can be artificially induced by radiation (for example, X-rays), but most natural topaz is not blue. Natural radiation in the rocks of Minas Gerais may have caused this.