Re: How can igneous granite can be transformed into garnet-mica schist?

The transformation to the igneous form (granite) is the simplest to 
explain.  To make granite from a schist of the same composition, you would 
simply melt the schist and then let it cool and crystallize slowly enough 
to get the coarse-grained texture characteristic of granites. To make a 
schist from a granite is a little more complex.  The primary mechanism is 
solid state recrystallization.  In this process, the ions that make up the 
crystals of the granite break free of their crystal structures, migrate 
along the edges of crystals and recrystallize into new minerals.  This 
rearangement is usually driven by changes in temperature or pressure.  If 
the pressure is greater from one direction than from the others, platy 
minerals like micas will tend to line up with their platy direction at 
right angles to the strongest pressure.  The alignment of these grains will 
give the rock a foliated texture (like a stack of paper or of leaves - a 
folio or foliage).  The metamorphic (changed) rock with a foliation is 
known as a schist.

How identical is "almost identical" is a very important question.  Granite 
is composed mostly of quarttz SiO2, alkali feldspar (K,Na)AlSi3O8, and 
plagioclase feldspar (Na,Ca)Al(Al,Si)Si2O8 with minor amounts of biotite 
K(Fe,Mg)3AlSi3O10(OH)2 and other Fe & Mg minerals (int hese formulas, 
elements listed inside parentheses can substitute for each other and may 
occur mixed together in the same crystal).  Garnet schist is composed 
primarily of biotite, muscovite KAl3Si3O10(OH)2, quartz, and garnet 
Ca3Al2Si3O12, (Fe,Mg)3Al2Si3O12, or Ca3Fe2Si3O12.  Obviously, the same 
basic elements are there, but the proportions can be problematic.  A 
typical schist will have more Al, more K, more OH (which is how water 
occurs in minerals) than a typical granite, and corresponingly less of 
everything else.  It may be possible to find granites and schists that 
really are identical, but it isn't the usual case.  In the usual case, some 
ions would have to be exchanged with the rock's surroundings via movement 
of ions into and out of fluids moving through the rock.  This process  is 
known as metasomatism.  Metasomatism is very commonly associated with 
metamorphism.  To turn a granite into a schist, metasomatism would add H2O, 
Al, and K and remove Ca and Na at the same time that the minerals were 
recrystallizing.

David Smith
Dept. of Geology and Environmental Science
La Salle University, Philadelphia, PA