QAPF Diagram (Igneous Rock Classification)

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 1)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 2)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 3)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 4)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 5)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 6)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

The QAPF Diagram: The Universal Language of Petrology

Imagine attempting to classify millions of different rocks found across the globe using just their physical appearance. Chaos would ensue. One geologist’s "granite" might be another geologist’s "diorite." To solve this, the International Union of Geological Sciences (IUGS) established the QAPF Diagram—the absolute, definitive classification scheme for all igneous rocks.

The QAPF diagram is not just a chart; it is a rigid mathematical grid based on the modal mineralogy (volume percentage) of a rock. By point-counting the minerals under a petrographic microscope and plotting them on this double-ternary diamond, a geologist generates a classification name that is universally recognized from universities in Tokyo to exploration camps in Canada.

Our interactive QAPF Calculator allows you to input your modal percentages, automatically normalizes the data, mathematically plots the coordinate on the canvas, and instantly outputs the official IUGS rock name. Below, we dive deep into how the diagram works, the incompatibility of certain minerals, and its limitations.

1. The Four Corners of the Diamond (Part 7)

The QAPF diagram is actually two ternary (triangular) diagrams welded together along their base. The four apices represent the four defining mineral groups:

• Q (Quartz): The pure silica endmember. Plotting high towards Q means the magma was overwhelmingly saturated in silica (SiO2).
• A (Alkali Feldspar): Includes orthoclase, microcline, and sanidine. High A indicates a potassium-rich (K-rich) melt.
• P (Plagioclase): Includes the solid-solution series from sodium-rich albite to calcium-rich anorthite.
• F (Feldspathoids): Also known as "foids" (e.g., nepheline, leucite). These minerals form when magma is severely starved of silica.

2. Why is it a Diamond? The Incompatibility Rule

You might wonder why we don't just use a single square or a tetrahedron to plot all four minerals. The answer lies in the fundamental laws of chemical thermodynamics: Quartz and Feldspathoids cannot coexist in the same rock.

If a magma contains both silica (SiO2) and feldspathoids (like nepheline, NaAlSiO4), they will instantly react with each other while the magma is still molten to form Alkali Feldspar (albite). Therefore, a rock will either have Quartz, or it will have Feldspathoids, but it will never have both in equilibrium. This chemical impossibility physically splits the diagram in half. The top triangle (Q-A-P) is for silica-oversaturated rocks (Granites, Diorites). The bottom inverted triangle (F-A-P) is for silica-undersaturated rocks (Syenites, Phonolites).

3. Plutonic vs. Volcanic Terminology

The geometry of the QAPF fields is identical for magma that cools slowly deep underground (Plutonic) and magma that erupts onto the surface (Volcanic). However, the names are completely different to reflect the texture of the rock.

For example, if you plot a rock with 35% Quartz, 40% Alkali Feldspar, and 25% Plagioclase, it falls squarely in Field 3. If that rock is coarse-grained (you can see the crystals), it is classified as a Granite. If that exact same magma erupted from a volcano and cooled rapidly into a fine-grained rock, it is classified as a Rhyolite. Our tool includes a simple toggle switch that automatically translates the geometry between Plutonic and Volcanic terminology.

4. Normalizing the Data (The Mathematics)

Plotting on a ternary diagram requires that the three variables add up to exactly 100%. Because rocks contain other minerals (like mafics), your raw Q, A, P, and F values will almost never sum to 100. You must normalize them.

For example, if your raw counts are Q=20, A=40, P=20, and Mafics=20: First, discard the mafics. Your new total is 20 + 40 + 20 = 80. To find the normalized Quartz value, divide Q by the total and multiply by 100: (20 / 80) × 100 = 25%. Our Javascript calculator handles this mathematical normalization entirely behind the scenes, allowing you to input raw point-count data directly.

5. Limitations of the QAPF Diagram

While the QAPF is the gold standard, it has a fatal flaw: it is completely useless for dark, mafic rocks. Because the diagram requires you to ignore mafic minerals, a rock that is 95% Pyroxene and 5% Plagioclase would be normalized to 100% Plagioclase, classifying it absurdly as an "Anorthosite" on the QAPF!

According to IUGS rules, if a rock contains more than 90% mafic minerals (Color Index > 90), it is an Ultramafic Rock and cannot be plotted on the QAPF. Instead, it must be plotted on a different ternary diagram (Olivine-Pyroxene-Hornblende). Similarly, Gabbros and Diorites plot in the exact same corner of the QAPF (Field 10) and must be distinguished based on the Anorthite content of their plagioclase, not by the QAPF alone.

6. Frequently Asked Questions (FAQ)

7. Authoritative References and Outbound Resources

• International Union of Geological Sciences (IUGS): The official governing body that created the QAPF classification rules. Visit IUGS.
• United States Geological Survey (USGS): For a massive database of rock classifications and geochemical data, visit the USGS.
• Geological Society of America (GSA): For educational resources on igneous petrology, visit the GSA.