Petrology is the study of rocks. Many people will say, "well yes, isn't that what geology is all about?" On the other hand if you tell people you collect minerals, they will think you have a "rock collection". These are ambiguities that are normal and expected from people just beginning to study geology.
Minerals are individual homogenous entities. A given mineral has one and only one organization of atoms (crystal structure). Rocks are composite entities. A rock can in a few cases consist of a single mineral species (such as marble consisting of calcite), but most rocks are an aggregate of several mineral species. In addition a rock is a unit that makes a significant part of the earth. This should hopefully clarify the difference between a rock and a mineral.
Every field has its own culture, history and spirit. This is quite definitely the case with petrology. There is a lot of tradition and some of it is disorganized clutter. Unlike mineralogy, there seems to be no need for any kind of central agreement to coin a new rock name, and there are lots of them. This is particulary true in the older literature, ut is by no means restricted to such. Along with a proliferation of names, there is no agreement about what defines commonly used rock names. One man's granite many not be the same as another man's. There may be some agreement these days, but only recently, so care must be taken in the literature. I found the following gem in one book:
No single classification of igneous rocks satisfies all petrographers. Indeed, the number of different classifications closely approaches the number of outstanding petrographers that have appeared since the start of the science.(Microscopic Petrography, page 18, 1956, E. Wm. Heinrich)
Other writers have attempted to justify this situation by pointing out that rigid classifications are deceiving. One rock grades imperceptibly into another. Nature is a continuum. This is certainly so, but if we are going to use names, we have to have some agreement if we are going to communicate. In practice though the inconsistency of classifications presents little actual difficulty. The proliferation of pointless names is merely an annoying nuisance.
Tom's Geology Info / [email protected]