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In 1976, a subcommission of the International Union of Geological Sciences (IUGS) proposed a classification scheme for plutonic and volcanic igneous rocks. As with most classification systems, this proposal sought to standardize the language spoken by petrologists, as many systems for naming igneous rocks were in limited, regional use at the time. This scheme is easy to understand, and now enjoys widespread acceptance. Most students of geology will recognize the ternary diagram (included below, in glorious ASCII) as a form of the QAP diagram.

The scheme requires that the modal composition be known for a particular rock sample. Usually, this is accomplished either by using a petrographic microscope to perform a point count or via instrumental analysis.

Minerals present in the rock are grouped into five main categories:

Q - quartz
A - alkali feldspar (An0-An5)
P - plagioclase feldspar (An5-An100)
F - feldspathoid minerals
M - mafic minerals (including, but not limited to pyroxene, amphibole, mica, sulfides, zircon, apatite etc.)
Rocks containing more than 90 per cent mafic minerals -- ultramafic rocks -- are classified using the ratios between the most abundant mafic minerals. These classification schemes will be covered in another write-up, and at a later date.

Rocks with less than 90 per cent mafic minerals can be plotted on the QAP/FAP diagram below. Quartz and feldspathoid minerals are mutually exclusive, allowing for the combination of the QAP and FAP ternary diagrams into one plot.

A few notes should be made about the following diagram:

  • The names for each of the rock types are arbitrary, and should only be used for qualitative description of a specific rock sample. In most igneous complexes, modal compositions may vary throughout a rock formation, depending on several dozen factors. Thus, formations are usually described as a range (i.e. "granite-granodiorite complex").
  • The description of a rock sample as found in the QAP/FAP diagram may be improved with a few qualifiers, usually the name of the dominant mafic mineral (i.e. "biotite granite" or "olivine muscovite granodiorite").
  • Some zones of the diagram show two or more names for a rock type. In this case, the determining factor may be the presence of a specific mineral, or in the case of diorite/gabbro/anorthosite, the calcium content of the plagioclase.
                                                  Q
                                                  *  100
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                                       **         A         **                                          
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                                  **                             **                                     
                                 **   10                     90  **     
                                *************************************  60                                 
                               **   **             **          **   **                                   
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                             **   **                *            **   **                                 
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                           **   **                  **             **   **                               
                          **   **                   **              **   **                              
                         **   **                     *               **   **                             
                        **   **                      *                **   **                            
                       ** B **           C          **        D       ** E **                           
                      **   **                        **                 **   **                           
                     **   **                          *                  **   **                          
                    **   **                           **                  **   **                         
                   **   **                            **                   **   **                        
                  **   **                              *                    **   **                       
                 **   **                               **                    **   **                      
                **   **                                **                     **   **              
               ***********************************************************************  20                  
             **   **                       *35         *65                    **   **                   
            **   **                       **            **                      **   **                   
           **   **                        **            **                       **   **                  
          **   **                         *              *                        **   **                 
         **   **                          *              *                         **   **                
        ** F **             G           **      H       **             I           ** J **               
       **   **                          **               **                          **   **              
    A**---**----------------------------*---------------*----------------------------**---** P  0            
       **   **                           **             **                           **   **               
        **   **                          **             **                          **   **               
         **   **                          *             *                          **   **                
          *******************************************************************************  10               
           **   **                               *50                             **   **                  
            **   **                              *                              **   **                   
             **   **                             *                             **   **                    
              **   **                            *                            **   **                     
               **   **                           *                           **   **                      
                **   **                          *                          **   **                      
                 **   **                         *                         **   **                       
                  **   **                        *                        **   **                        
                   **   **                       *                       **   **                         
                    **   **                      *                      **   **                          
                     ** K **           L        *         M           ** O **                           
                      **   **                    *                    **   **                            
                       **   **                   *                   **   **                             
                        **   **                  *                  **   **                              
                         **   **                 *                 **   **                              
                          **   **                *                **   **                               
                           **   **               *               **   **                                
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                               *************************************  80                                  
                                **                               **        
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                                      **         O         **                                          
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                                                 *  100
                                                F  
              


     Plutonic rocks                         Volcanic rocks
A.   quartzolite                            (n/a)      
B.   alkaline feldspar granite              alkaline feldspar rhyolite
C.   granite                                rhyolite
D.   granodiorite                           dacite
E.   tonalite                               plagiodacite
F.   alkaline feldspar syenite              alkaline feldspar trachyte
G.   syenite                                trachyte
H.   monzonite                              latite
I.   monzodiorite/monzogabbro               latite-andesite/mugearite
J.   diorite/gabbro/anorthosite             andesite/basalt 
K.   feldspathoid syenite                   phonolite
L.   feldspathoid monzonite                 tephritic phonolite
M.   essexite                               phonolitic tephrite
N.   theralite                              tephraite/basanite
O.   foidite                                foidite/nephelinite/leucitite
The above names for volcanic rocks should only be used if a modal analysis of the rock can be completed. As many volcanic rocks are too fine-grained or vitreous to be point-counted, those rocks are named according to texture.


Sources:
Simon & Schuster's Guide to Rocks and Minerals
Principles of Igneous and Metamorphic Petrology (1990), Anthony R. Philpotts, 498 pp., Prentice Hall

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