How to Use QAPF Diagram to Classify Igneous Rocks?

QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for classification in QAPF diagram. Q, A, P and F percentages are normalized (recalculated so that their sum is 100%).

Igneous rocks are classified on the basis of mineralogy, chemistry, and texture. Texture is used to subdivide igneous rocks into two major groups:

(1) the plutonic rocks, with mineral grain sizes that are visible to the naked eye,

(2) the volcanic rocks, which are usually too fine-grained or glassy for their mineral composition to be observed without the use of a petrographic microscope.

As noted in the sidebar to the left, this is largely a genetic classification based on the depth of origin of the rock (volcanic at or near the surface, and plutonic at depth). Remember that porphyritic rocks have spent time in both worlds. Let's first examine the classification of plutonic rocks.

How to Use QAPF Diagram to Classify Igneous Rocks?

To use the QAPF diagram, you first need to determine the percentage of each mineral in the rock. This can be done by performing a petrographic analysis of the rock. Once you have the percentages of each mineral, you can then plot them on the QAPF diagram.

The QAPF diagram is divided into four quadrants:

Quartz (Q): Rocks that fall in this quadrant are high in quartz and low in other minerals. These rocks are typically felsic and intrusive in origin.

Alkali feldspar (A): Rocks that fall in this quadrant are high in alkali feldspar and low in other minerals. These rocks are typically felsic and extrusive in origin.

Plagioclase feldspar (P): Rocks that fall in this quadrant are high in plagioclase feldspar and low in other minerals. These rocks are typically mafic and intrusive in origin.

Ferromagnesian minerals (F): Rocks that fall in this quadrant are high in ferromagnesian minerals and low in other minerals. These rocks are typically mafic and extrusive in origin.

QAPF diagrams are mostly used to classify plutonic rocks (phaneritic rocks), but are also used to classifyvolcanic rocks if modal mineralogical compositions have been determined. QAPF diagrams are not used to classify pyroclastic rocks or volcanic rocks if modal mineralogical composition is not determined, instead the TAS classification (Total-Alkali-Silica) is used.

TAS is also used if volcanic rock contains volcanic glass (such as obsidian). QAPF diagrams are also not used if mafic minerals make up more than 90% of the rock composition (for example:peridotites and pyroxenites).

An exact name can be given only if the mineralogical composition is known, which cannot be determined in the field.

How to Use QAPF Diagram to Classify Igneous Rocks?

Using QAPF diagram

To use a QAPF diagram to classify igneous rocks, follow these steps:

Determine the modal mineralogy of the rock. This can be done by counting the points of different minerals in a thin section of the rock. The modal mineralogy is expressed as a percentage of the total rock volume.
Plot the modal mineralogy on the QAPF diagram. The corners of the QAPF diagram represent quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoid (F). The length of each side of the triangle is divided into 100 equal parts.
Normalize the modal mineralogy. This means that the sum of Q, A, P, and F must be equal to 100%. This is done by dividing each mineral percentage by the sum of all four percentages and multiplying by 100.
Determine the rock name. The rock name is determined by the position of the normalized modal mineralogy on the QAPF diagram.

There are many different rock types, but some of the most common include:

Granite: Q > 20%, P > 65%
Granodiorite: Q > 20%, P < 65%
Tonalite: Q < 20%, P > 65%
Diorite: Q < 20%, P < 65%
Gabbro: P < 5%, F < 5%
Basalt: P < 5%, F < 5%
Nepheline syenite: F > 5%

The plotting of rock modes on these triangular diagrams is simpler than it may appear. The three components, Q (quartz) + A (alkali (Na-K) feldspar) + P (plagioclase), are recalculated from the mode to sum to 100 percent. Each component is represented by the corners of the equilateral triangle, the length of whose sides are divided into 100 equal parts.

Any composition plotting at a corner, therefore, has a mode of 100 percent of the corresponding component. Any point on the sides of the triangle represents a mode composed of the two adjacent corner components. For example, a rock with 60 percent Q and 40 percent A will plot on the QA side at a location 60 percent of the distance from A to Q.

A rock containing all three components will plot within the triangle. Since the sides of the triangle are divided into 100 parts, a rock having a mode of 20 percent Q and 80 percent A + P (in unknown proportions for the moment) will plot on the line that parallels the AP side and lies 20 percent of the distance toward Q from the side AP.

If this same rock has 30 percent P and 50 percent A, the rock mode will plot at the intersection of the 20 percent Q line described above, with a line paralleling the QA side at a distance 30 percent toward P from the QA side. The third intersecting line for the point is necessarily the line paralleling the QP side at 50 percent of the distance from the side QP toward A.

A rock with 25 percent Q, 35 percent P, and 40 percent A plots in the granite field, whereas one with 25 percent Q, 60 percent P, and 15 percent A plots in the granodiorite field. The latter is close to the average composition of the continental crust of the Earth.

How to Use QAPF Diagram to Classify Igneous Rocks?

Using QAPF diagram

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