MadSci Network: Earth Sciences

Re: Why are granitic rocks light in color?

Date: Sat Jan 19 19:16:48 2002
Posted By: Matthew Buynoski, Senior Member Technical Staff,Advanced Micro Devices
Area of science: Earth Sciences
ID: 1010714546.Es


"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         
                        diorite                         andesine
    middle                                  amphiboles               
                        granodiorite                    oligoclase
    late                                                albite
                        granite                         potassium-rich
    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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