Re: Number of rocktypes

Concur with George on the number of minerals.

Rock Types? How about 2 Rock types: Earth Rocks and non-Earth Rocks.

Actually rocks can be 'typed' by either origin, texture, or composition or some combination of the three.

The igneous, sedimentary, metamorphic divisions still work pretty well if you count only the most recent exposure to heat and pressure (i.e., earth magmatism, erosion/chemical precipitation then resolidification, and transformation of type 1 or type 2 via non-complete remelt and/or intense pressure. This works for 95% of all stone out there, but the remaining 5 percent can be tricky, since they are things like water-laid tuff (igneous or sedimentary?), heat-based precipitates in igneous, ashfall which is only imperfectly remelted to stone--chilled margin metamorphic alteration of igneous...etc. Luckily most of these aren't very common occurrences.

Rocks typed by composition seriously depends if you are lumper or a splitter. When does a porphyritic granite grade from rhyolite porphyry?
How much 'order' does a graphic granite need to be to be considered a gneiss? Just how much 'mafic' mineral does dark rhyolite have to have to be considered a rhyodacite, or a rhyodacite into a true dacite? And the mafic sequences have similiar transition zones, though I'm not as familiar with them. Heck, just look at the whole range of minerals in the feldspar family, and how one grades into another. Yes, there are certain percentages of X mineral which are required for rock transitions, but how many samples do you have to take to figure out if it qualifies, since every individual rock fragment is liable to describe a bell curve around some mean, and not be exactly identical. And when do you count impurities, and when do you ignore them? I case could probably be made that in most cases and with appropriate levels of measurement precision, no two rock fragments are exactly alike (except if they are indeed 100% of some mineral (clear quartz xl.)

So while petrologists can group rocks by origin,texture and composition, there is always likely an exception out there to prove that "every" granite is pink, or "every" basalt is cryptocrystalline, or "every" diabase has crystals of a prescribed size. This doesn't even get into the fact that certain rocks have multiple names which are more or less descriptive of them..(I know a black rock with white phenocrysts which weathers 'blue' (actually a sort of gunmetal gray) which is called dellenite, rhyodacite, andesite, black granite and blue granite. I've been in heated roadside discussions in which some insisted a rhyolite porphyry ash-fall tuff was actually ignimbrite. So go look up ignimbrite, if you don't believe em.

But it sure is fun, but I'm not sure anyone can say there are 'X rock types' with any sort of final authority, the same way one can do a chemical or crystal test and tell muscovite from biotite, or quartz from calcite.
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