|MadSci Network: Earth Sciences|
Hello! "Granite" or "granitic rock" is one type of rock that crystallizes from a melt at great depth. The general term is "plutonic" rock. Geologists have devised an intricate set of definitions for each type of plutonic rock; there are lots of them (both names and rocks). What most people refer to as "granite" is generally rock that tends to crystallize last, or near to last, out of the original melt. It turns out that most of the minerals (e.g. pyroxenes, amphiboles) that crystallize early are darker colored, and use up most of the iron and magnesium that was originally in the melt. As the molten mass continues to cool, it progressively becomes enriched in those elements that crytallize into quartz, sodium- and potassium-rich feldspars, and certain micas (e.g. muscovite). All this is summarized in Bowen's Reaction Series, which goes like this Crystallization Rock Temper- Other Feld- Sequence Type ature Minerals spars -------------------------------------------------------------------- start basalt high olivene bytownite gabbro labradorite pyroxenes diorite andesine middle amphiboles granodiorite oligoclase biotite late albite granite potassium-rich muscovite quartz last pegmatite low many rare pegmatite minerals Granite is thus made mostly of quartz, feldspars, and muscovite mica, often with a smattering of some other minerals. It turns out that quartz is pretty much white or clear. The late-crystallizing feldspars (albite and/or potassium-rich feldspar) are usually white, very light tan, or pinkish. The muscovite is somewhat darker (can be light brown, or greenish, or even have a light purplish color depending on minor impurities). All of this leads to granite being a rather pastel rock overall. The book I used is Press and Sevier, "Earth", chapter 16. At your grade level, you will need some help on the chemistry reading a book like that (it's an introductory college text) but you will be able to understand most of it. Many other introductory geology texts also discuss the same material. You could also try G. W. Robinson's book, "Minerals", which presents rock formation at a less technical level, and has a lot of really gorgeous photographs of minerals in it as a bonus. One last note. You may find that the use of the term "granite" in everyday life is not necessarily consistent with the scientific definition. For instance, some quite dark plutonic rock is quarried, shaped, polished and sold for kitchen countertops as "black granite".
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