Polymorphism: One Chemical, Multiple Minerals.

     I have a few minerals that I’d like to blog about in the future that are polymorphs, so I figured that it would be a good idea to post a blog about polymorphs first. Polymorphs are simply minerals that have the same chemical makeup, but different crystal structures. For example, the chemical titanium dioxide can actually form three different minerals: brookite, rutile, and anatase. Rutile has prismatic tetragonal crystals, anatase has tabular (flat) tetragonal crystals, and brookite has orthorhombic crystals, yet all three are the same chemical. The chemical makeup of a mineral (its molecules) can determine a great deal concerning the mineral's properties, but the way in which these molecules are arranged when the substance crystalizes can also affect them as well. Think of it this way: if you have three red Lego blocks and four blue ones you can build any number of different objects with them, yet they are the same three reds and four blues. The same thing is happening in polymorphism: one type of molecule might usually arrange itself in one way, giving the crystal certain properties; but it can also arrange itself in another way, giving the mineral a whole new range of properties.
 How does this happen? The way that the chemicals that make up polymorphous minerals crystallize depends a lot on their surroundings. Heat and pressure have a lot to do with what polymorph is formed (by the way, the term “polymorph” also refers to each form of the chemical. For example calcite and aragonite are both polymorphs of calcium carbonate.) There is a great deal more science that could be delved into here, but for now I’ll leave it out.
     Sometimes certain polymorphs of the same chemical are less stable than the other. Anatase and brookite both become rutile when subjected to intense heat, and aragonite will slowly revert to calcite over time and will do so rapidly when heated.
     I hope that the subject of polymorphism in minerals was as interesting to you as it was to me. I also hope to make some posts about a few polymorphous minerals soon!