Crystals in Rocks

Q: My science teacher and I had a conversation about the crystal formation  on rocks and we got confused. Do crystals form on igneous rocks or  do they form on metamorphic rocks?  - Justin W

A: Crystals form in both kinds of rock. 

A way to think of crystal formation is to envision a crystal mush: As the intruded magma slowly cools (slowly because if it is underground it is well insulated), crystals will begin to form. The slower the cooling, the larger the crystals, in general. Some crystals will settle to the bottom of the crystal-magma mush if they are denser - and if there is no circulation happening in the crystal mush. These precipitated-out crystal accumulations can sometimes be seen in some ultramafic bodies exposed by later weathering - this bottom layer looks like a mat of "crystal toothpicks."  

More commonly, however, the crystal mush is  very active - convecting or circulating with repeated injections of magma from depth, and/or gas coming out of solution - until the percentage of crystals is too great for further circulation. As crystals continue to form, the percentage of the fluid decreases until the entire intrusive body is solidified. Often in late stages of this crystalization process, cracks will form in the intrusive body itself and in the surrounding host rock, and these cracks will fill with the last bits of fluid in the crystal mush, forming veins. Because there is a preferred order of crystal growth, the last-gasp fluid tends to be different from the average composition of the original magma body that entered the crust from the mantle in the first place.

With metamorphic rocks it's a bit different, because the material was solid to begin with, but under deep tectonic or sedimentary burial (or contact with a hot intruding body), the original material (which could be sediments, or could be older intrusive rocks) heats up and partially melts. Then something called recrystalization takes place. 

If you are ever in Tucson, Arizona, look north towards the Catalina mountains. From a distance you can clearly see the original sedimentary layering, but these rocks have been buried at least 15 kilometers deep and then uplifted by tectonic processes. When you get up close, you will see that coarse crystals have formed during this burial-heating process, so it looks more like a granite than a sedimentary rock. It's actually called an "augen gneiss", words derived from the German language where this sort of rock was first described. Even more fascinating is that as you walk farther north in the Catalina mountain complex, the augen gneiss gradually becomes a classic granite. This means that the more northern sedimentary rocks were buried even deeper. Old time miners would say that these rocks were "stewed and cooked."

It's a lot more complicated than this, of course, because there is heat and fluid released when crystals form. There is also contact metamorphism, where a hot intruding body will heat up the edges of the surrounding rocks and change them chemically both via heat and via fluid and chemical transfer across the boundary. 

I hope this answers your questions. 

==Jeff Wynn