Enter Mineral Percentages

Enter the percentages of each mineral group in your rock sample:

Diagram

Q1 F1 F2 Q2 Q3 Q4 Q5 granite foidolite quartzsyenite quartzmonzonite foid-bearingsyenite foidmonzosyenite foid-bearingmonzonite syenite monzonite quartz-richgranitoid granodiorite tonalite alkali feldspar granite foid syenite 60 20 10 10 35 50 65 90 90 5 10 60 90 F1. foid-bearing alkali feldspar syeniteF2. foid-bearing diorite/gabbro Q1. alkali feldspar syeniteQ2. quartz alkali feldspar syeniteQ3. diorite/gabbroQ4. quartz diorite/gabbroQ5. quartzolite foid diorite foidmonzodiorite quartzmonzodiorite foid-bearingmonzodiorite monzodiorite Classification of Igneous RocksQ-A-P-F Q F A P Q = quartzA = alkali feldsparP = plagioclase feldsparF = feldspathoid (foid)

How to Use the QAPF Diagram

The QAPF diagram presents the proportions (ratios) of four plutonic mineral(s) or mineral groups:

  • Q: Quartz
  • A: Alkali feldspars
  • P: Plagioclase feldspars
  • F: Feldspathoids (foids)

Because F and Q groups cannot simultaneously form in plutonic rocks—due to the difference in their respective silica contents—the QAPF diagram is drawn as two mutually exclusive triangle plots, i.e., QAP and FAP. These are joined along one side such that, between them, each of the two triangle plots exclude either the Q group or F group minerals.

Diagram for naming plutonic/phaneritic igneous rocks with An<50%

This QAPF diagram is two independent ternary diagrams that share the A-P edge. The upper triangle has the corners A-Q-P, where A is Alkali feldspar, Q is Quartz, and P is Plagioclase. The lower triangle has the corners A-P-F, where A is Alkali feldspar, P is Plagioclase, and F is Foid (meaning any feldspathoid mineral). Numbers on horizontal lines on the chart are ternary modal (=volume) percentage (Q relative to A+Q+P or F relative to A+P+F). Numbers on inclined or vertical lines are binary modal percentage (P relative to A+P).

Diagram for naming plutonic/phaneritic igneous rocks with An>50%

This QAPF diagram is two independent ternary diagrams that share the A-P edge. The upper triangle has the corners A-Q-P, where A is Alkali feldspar, Q is Quartz, and P is Plagioclase. The lower triangle has the corners A-P-F, where A is Alkali feldspar, P is Plagioclase, and F is Foid (meaning any feldspathoid mineral). Numbers on horizontal lines on the chart are ternary modal (=volume) percentage (Q relative to A+Q+P or F relative to A+P+F). Numbers on inclined or vertical lines are binary modal percentage (P relative to A+P).

Anorthite Content

When the anorthite content (An) is greater than 50%, the following name changes apply:

  • quartz monzodiorite → quartz monzogabbro
  • quartz diorite → quartz gabbro
  • monzodiorite → monzogabbro
  • diorite → gabbro/anorthosite
  • foid-bearing diorite → foid-bearing gabbro
  • foid diorite → foid gabbro
  • foid-bearing monzodiorite → foid-bearing monzogabbro
  • foid monzodiorite → foid monzogabbro

Calculation Method

To use this classification method:

  1. The concentrations (the modes) of the four mineral groups must be determined or estimated.
  2. These values are then normalized to 100%.
  3. For example, for a rock identified as having 20% mica, 30% quartz (Q), 30% alkali feldspar (A), and 20% plagioclase (P), the mica is disregarded.
  4. The normalized ratios (proportions) of the Q, A, and P groups are calculated as 37.5%, 37.5% and 25% = 100%.
  5. Of these, the (again) normalized relative proportions of A and P are 37.5/62.5 = 60% and 25/62.5 = 40%.
  6. The rock can now be plotted on the diagram by finding a horizontal line representing 37.5% quartz and then plotting a point on it 60% of the way across from the A side to the P side.
  7. For this example, the rock can be classified as a Monzogranite.