QAPF diagram

The QAPF diagram is a standardized classification tool for igneous rocks, particularly plutonic varieties, based on the modal (volume) percentages of four key mineral components: quartz (Q), alkali feldspar (A), plagioclase feldspar (P), and feldspathoids (F). It consists of two adjoining ternary diagrams forming a diamond-shaped plot—the upper Q-A-P triangle for quartz-bearing rocks and the lower A-P-F triangle for feldspathoid-bearing rocks—divided into 15 fields that assign specific names to rocks with less than 90% mafic minerals, such as granite, syenite, and gabbro. Developed by geologist Albert Streckeisen in 1976 and formally recommended by the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks, the diagram provides a modal mineralogical framework to ensure consistent nomenclature across global geological studies.^[1]^[2] This classification applies primarily to phaneritic (coarse-grained) rocks where minerals are visible and can be quantified, though a modified version exists for aphanitic (fine-grained) volcanic equivalents like rhyolite and trachyte, using similar field boundaries but adjusted for texture. To plot a sample, the percentages of Q, A, P, and F are normalized to 100% after excluding mafic minerals, with quartz and feldspathoids treated as mutually exclusive; additional qualifiers like leucocratic (light-colored, low mafics) or melanocratic (dark, high mafics) refine names based on color index. The diagram's fields incorporate boundaries derived from mineral stability and composition, such as the 5% quartz threshold separating alkali feldspar granite from quartz syenite, promoting precise identification without reliance on chemical analyses alone. Limitations include its inapplicability to ultramafic rocks (>90% mafics) or those requiring normative (calculated) mineralogy, where supplementary schemes like the total alkali-silica (TAS) diagram are used.^[2]^[3] Adopted since the 1970s, the QAPF system has become foundational in petrology, facilitating comparisons of igneous suites worldwide and integrating with broader IUGS frameworks for volcanic, pyroclastic, and carbonatite rocks. Its emphasis on observable mineral modes underscores the importance of fieldwork and microscopy in rock identification, while software tools like Auto-QAPF aid in automated plotting and verification. Ongoing refinements by IUGS ensure the diagram evolves with petrographic advances; a preliminary report in 2024 proposed minor updates, such as replacing "mafic" (M) with "X" for non-QAPF minerals and removing scapolite from the plagioclase field, while preserving the core structure.^[3]^[4]

Background

Definition and Purpose

The QAPF diagram is a modal classification system for igneous rocks, utilizing the relative proportions of four primary minerals: quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F).^[5] It employs a double ternary diagram structure, comprising the QAP triangle (plotting Q, A, and P) and the FAP triangle (plotting F, A, and P), which share the A-P axis, with mineral percentages normalized to sum to 100% after excluding mafic minerals and accessory phases.^[6] This approach allows for the visual plotting of rock compositions to assign standardized names based on predefined fields within the diagram.^[5] The diagram's purpose is to enable rapid, objective identification of felsic to intermediate plutonic rocks exhibiting phaneritic textures, distinguishing types such as granite (high Q and A), syenite (dominant A with minimal Q), and gabbro (prevalent P) through mineral mode analysis alone, bypassing the need for geochemical data.^[6] Developed by the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks, it promotes uniform nomenclature across petrological studies and facilitates comparisons in geological mapping and research.^[5] At its core, the QAPF diagram relies on modal mineralogy, determined by point-counting or visual estimation of mineral volumes in thin sections, focusing solely on the QAPF components while disregarding mafic minerals like pyroxene or olivine.^[6] It is designed for rocks with less than 90% mafic mineral content (equivalently, more than 10% total QAPF minerals), rendering it unsuitable for ultramafic varieties.^[5] For volcanic rocks, the QAPF diagram applies where modal mineralogy can be determined, including aphanitic textures with visible phenocrysts. When direct modal analysis is impractical due to fine grain size or glassy texture, the chemical-based Total Alkali-Silica (TAS) diagram is recommended instead.^[7]

Historical Context

The development of the QAPF diagram emerged from efforts to establish a unified international system for classifying igneous rocks, addressing longstanding inconsistencies in nomenclature that hindered global geological collaboration. Prior systems, such as the quantitative chemico-mineralogical classification proposed by Cross, Iddings, Pirsson, and Washington in 1903, relied heavily on chemical norms but often led to ambiguous rock names due to variations in analytical methods and interpretations. Similarly, Albert Johannsen's modal classification from the 1910s to 1930s, while emphasizing mineral proportions, introduced excessive complexity with over 1,000 potential names, exacerbating discrepancies across national traditions. These issues became particularly acute after World War II, as increased international fieldwork and data sharing among geologists underscored the need for standardized terminology to facilitate cross-border research and mapping.^[8]^[9] In response, Albert Streckeisen initiated a comprehensive review of igneous rock classification starting in 1958, culminating in the formation of the IUGS Subcommission on the Systematics of Igneous Rocks following the 23rd International Geological Congress in Prague in 1968. The subcommission, with Streckeisen as chair, began formal work in 1970 under the IUGS Commission on Petrology, aiming to create a modal-based scheme prioritizing essential minerals for practical field use. Initial proposals for the QAPF diagram—a ternary plot using quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoid (F) modes—were presented at the subcommission's 1972 meetings in Bern and Montreal, where key recommendations were agreed upon. The diagram was first published in 1974 as part of the subcommission's guidelines for plutonic rocks, with refinements in 1976 that clarified boundaries and naming conventions, informally dubbing it the "Streckeisen classification."^[9]^[8]^[1] The QAPF diagram achieved official IUGS endorsement in 1989 through the subcommission's comprehensive recommendations, compiled in a glossary that integrated it with complementary schemes for volcanic and ultramafic rocks. Subsequent minor updates in 1991 addressed procedural details, while the 2002 revision by R.W. Le Maitre and colleagues incorporated feedback on handling feldspathoids in foid-bearing assemblages, ensuring broader applicability without altering the core diagram. This evolution reflected over three decades of iterative refinement, solidifying the QAPF as a cornerstone of modal classification for felsic to intermediate plutonic rocks.^[10]

Diagram Components

Axes and Mineral Parameters

The QAPF diagram is structured as a double-triangle plot, comprising two independent ternary diagrams that share the A-P edge. The upper triangle represents silica-saturated rocks and is defined by the QAP axes, where the sum of quartz (Q), alkali feldspar (A), and plagioclase (P) is normalized to 100%. The lower triangle accommodates silica-undersaturated rocks via the FAP axes, with feldspathoids (F), alkali feldspar (A), and plagioclase (P) summing to 100%. This shared A-P edge allows for a continuous representation of compositions across the silica saturation spectrum, as adopted in the International Union of Geological Sciences (IUGS) classification system.^[1] The mineral parameters are modal percentages determined by volume, excluding mafic minerals (such as pyroxene, olivine, and amphibole) and accessory phases (e.g., biotite, hornblende, and opaque oxides), which are omitted to focus on the felsic framework. Q denotes quartz, ranging from 0 to 100% in the QAP triangle; A includes alkali feldspars like orthoclase, microcline, and sanidine; P encompasses the plagioclase series from albite (sodic) to anorthite (calcic); and F represents feldspathoids such as nepheline, leucite, sodalite, and analcime. These parameters are normalized such that, for QAP plots, the proportions are Q/(Q+A+P) × 100, F/(F+A+P) × 100, and P/(A+P) × 100, ensuring the points lie within the respective triangles.^[11] A key feature is the mutual exclusivity of Q and F, stemming from their inverse relationship tied to silica saturation: significant quartz indicates silica excess, precluding stable feldspathoids, while abundant feldspathoids signal silica deficiency, rendering quartz unstable. Thus, rocks plot exclusively in either the QAP or FAP triangle, with negligible overlap. The diagram includes a central division line at 5% Q (upper) or 5% F (lower) to separate oversaturated from undersaturated compositions. Fields within each triangle are delineated by contour lines at 10% intervals, facilitating transitions such as regions where Q ranges from 5% to 20% and P exceeds A by varying degrees.^[1]^[12]

Fields and Rock Names

The QAPF diagram for plutonic rocks delineates 15 primary fields based on the normalized modal percentages of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F), with names assigned according to International Union of Geological Sciences (IUGS) conventions for rocks containing less than 90% mafic minerals (M). Field boundaries are established at key percentage contours, including 5%, 10%, 20%, 35%, and 65%, along with a 50% division on the A-P edge to distinguish A > P, A ≈ P, and P > A compositions. These fields provide a visual taxonomy that correlates mineral proportions with traditional rock names, emphasizing the felsic components while accounting for silica-oversaturated (Q-bearing) or undersaturated (F-bearing) assemblages. In the QAP triangle, which applies to quartz-bearing rocks (F ≈ 0), the fields are positioned relative to the Q apex and the A-P base. For example, the granite field occupies the area where Q > 20% and A > P, reflecting a dominance of quartz and alkali feldspar typical of silica-rich, potassic intrusions; adjacent is alkali-feldspar granite for higher A content. The granodiorite field is where Q > 20% and P > A. The quartz monzonite field spans Q = 5–20% with A ≈ P, indicating balanced feldspars with moderate quartz; quartz syenite occupies Q = 5–20% and A > P, while quartz diorite is Q = 5–20% and P > A. Further toward the P apex, for Q = 0–5% and P > A with A significant (typically 10–35%), monzodiorite is named, transitioning to diorite (A < 35%, P > 65%) and gabbro (P > 65%, Q and A < 5%). Tonalite names the area where P > A and Q < 20%, typically associated with sodic to intermediate plagioclase. Without quartz (Q ≈ 0), the syenite field lies where A > P, monzonite where A ≈ P, and diorite where P > A, with gabbro for the P-dominant region (P > 65%). The adjoining FAP triangle accommodates foid-bearing rocks (Q ≈ 0), sharing the A-P edge and using F as the third apex for silica-undersaturated compositions. Here, the nepheline syenite (or foid syenite) field is defined by F > 10% and A > P, highlighting alkali-rich, undersaturated plutonics. The foid monzosyenite field occurs at F > 10% and A ≈ P, while foid monzodiorite spans F > 10% and P > A (with A significant). Other fields include foid diorite (P > A, low A) and foidolite (F > 60%). Transitions to foid-bearing variants (e.g., foid-bearing syenite) are denoted by adding "foid-bearing" prefixes to QAP names when F = 2–10%, bridging the two triangles. Ultramafic rocks (M > 90%) fall outside these fields and employ a separate ultramafic triangle based on olivine, pyroxene, and hornblende modes.

Triangle	Representative Field	Boundary Conditions	Rock Name
QAP	High Q, A > P	Q > 20%, A > P	Granite
QAP	Moderate Q, A ≈ P	Q = 5–20%, A ≈ P	Quartz monzonite
QAP	Low Q, P > A (intermediate)	Q = 0–5%, P > A, A ≈ 10–35%	Monzodiorite
QAP	No Q, P >> A	Q ≈ 0, P > 65%	Gabbro
QAP	Low Q, P > A	Q < 20%, P > A	Tonalite
FAP	High F, A > P	F > 10%, A > P	Nepheline syenite
FAP	Moderate F, A ≈ P	F > 10%, A ≈ P	Foid monzosyenite
FAP	Moderate F, P > A	F > 10%, P > A	Foid monzodiorite

Classification Process

Normalization of Mineral Modes

The normalization of mineral modes for the QAPF diagram begins with determining the modal percentages of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F) through point counting or visual estimates under a microscope.^[1] These percentages exclude mafic minerals and accessory phases, which are ignored in the calculation to focus on the felsic components.^[1] The sum of Q, A, P, and F (denoted as QAPF total) is then computed, and each mineral mode is recalculated as a proportion of this total to ensure the values sum to 100% for plotting.^[13] The normalization formula is given by:

\text{Normalized } Q = \left( \frac{Q}{Q + A + P + F} \right) \times 100

Similar expressions apply to A, P, and F.^[1] This procedure applies to rocks where mafic minerals constitute less than 90% of the mode, as higher mafic contents shift classification to ultramafic categories outside the QAPF framework.^[13] Quartz and feldspathoids are mutually exclusive in most igneous rocks, so samples typically contain significant amounts of either Q or F, but not both; minor amounts (<5%) of one are set to zero for plotting. For plotting, if normalized F < 5%, set F=0 and renormalize Q, A, P to 100% for the Q-A-P triangle; conversely, if normalized Q < 5%, set Q=0 and renormalize F, A, P to 100% for the F-A-P triangle.^[1]^[7] Consider a sample with raw modal data: Q = 15%, A = 20%, P = 30%, F = 2%, and mafics = 33%. The Q + A + P total is 65% (since F < 5%, exclude F), so the normalized values for the QAP triangle are Q = (15/65) × 100 ≈ 23.1%, A = (20/65) × 100 ≈ 30.8%, and P = (30/65) × 100 ≈ 46.2%.^[14]^[7] This adjustment ensures the coordinates sum to 100% on the chosen triangle, facilitating precise placement within the diagram's fields for rock naming.^[1] For volcanic rocks, modal data from phenocrysts and groundmass is preferred when available, as it provides direct mineralogical insight; however, if modes cannot be reliably estimated due to fine grain size or alteration, chemical proxies such as the total alkali-silica (TAS) diagram are used instead.^[13]

Step-by-Step Interpretation

The interpretation of the QAPF diagram begins with normalized modal mineral percentages of quartz (Q), alkali feldspar (A), plagioclase (P), and feldspathoids (F), which serve as prerequisites for plotting. Quartz and feldspathoids are mutually exclusive in most igneous rocks, so samples typically contain significant amounts of either Q or F, but not both. First, determine silica saturation to select the appropriate triangle: if normalized Q ≥ 5%, set F=0 and plot on the QAP triangle; if normalized F ≥ 5%, set Q=0 and plot on the FAP triangle. If both are <5%, use the QAP triangle.^[7] Next, plot the normalized coordinates on the selected ternary triangle, where each apex represents 100% of one component (Q, A, P for the QAP triangle; F, A, P for the FAP triangle), and the sides are divided into 10% increments for precision. Locate the position by drawing lines parallel to the sides from the percentage values—for instance, from the Q apex for the Q percentage, intersecting with lines from A and P. Compare the point to the diagram's numbered fields, which are bounded by specific lines (e.g., the M curve separating monzonic from dioritic compositions based on P/(A + P) ratios).^[7] Assign the root name based on the enclosing field, such as granite (field 2–3) or syenite (field 7). For example, a point at normalized 30% Q, 20% A, and 50% P on the QAP triangle falls within field 10, designating the rock as quartz diorite. Similarly, normalized 10% Q, 40% A, and 50% P places it in field 9, classifying it as monzodiorite. For undersaturated rocks, normalized 20% F, 30% A, and 50% P on the FAP triangle corresponds to field 13, naming it foid monzodiorite.^[7] Within fields, interpolate for modifiers if a component deviates notably; for instance, low Q (5–10%) in the granite field yields quartz-poor granite. The 10% rule applies for hybrid names when a secondary component ranges from 10–60%, such as quartz alkali feldspar syenite if Q is 10–20% in the syenite field. The diagram assumes phaneritic textures typical of plutonic rocks, and misinterpretation can occur if modal percentages are inaccurate due to sampling or identification errors.^[7]

Applications and Limitations

Use in Igneous Rock Classification

The QAPF diagram serves as the primary tool for classifying plutonic igneous rocks during geological mapping, particularly for identifying common lithologies such as granites within orogenic belts like the Himalayan or Variscan ranges. In fieldwork, geologists integrate QAPF-based nomenclature with visual estimates from hand samples and point-counting from thin sections to achieve rapid, standardized identification of rock types, facilitating efficient documentation of intrusive suites in regional surveys.^[15]^[16] In petrological research, the diagram aids in tracking magmatic evolution by plotting differentiation trends in QAPF space, revealing fractional crystallization paths from mafic to felsic compositions, as observed in studies of calc-alkaline intrusions. It supports provenance analyses by correlating modal compositions with potential source regions or parental magmas, and its integration into databases like GEOROC enables global comparisons of igneous suites for tectonic reconstructions.^[16]^[17] For volcanic rocks, which lack easily measurable modes due to fine grain size, the QAPF diagram applies estimated mineral proportions derived from geochemical approximations or phenocryst counts to classify extrusive equivalents of plutonic rocks. It is often combined with geochemical plots, such as TAS diagrams, to provide hybrid classifications that account for both modal and chemical data in hybrid or altered samples. Post-2010 digital tools, including IgPet software, automate QAPF plotting from modal or normative data, enhancing precision in large datasets.^[15]^[18]^[19] As an essential component of IUGS-recommended workflows for modal classification, the QAPF diagram has been applied in studies of alkaline provinces, such as the Kola Peninsula syenites, where it delineates peralkaline trends from nepheline syenite to foid-bearing variants in the Khibina and Lovozero complexes.^[15]^[20]

Constraints and Complementary Methods

The QAPF diagram exhibits several key constraints that restrict its use in igneous rock classification. It requires accurate modal mineral proportions, rendering it ineffective for aphyric volcanic rocks lacking discernible modes due to their fine-grained, glassy, or porphyritic textures, as well as for pyroclastic deposits where fragmentation and alteration hinder reliable point counting or visual estimation.^[7] The diagram is also inapplicable to ultramafic rocks containing more than 90% mafic minerals (M > 90%), as these fall outside the QAPF framework focused on felsic components.^[7] Furthermore, its reliance on mineral modes limits its ability to distinguish chemical variations; for instance, rocks from distinct magmatic series, such as tholeiitic and alkaline basalts, may occupy the same QAPF field despite differing geochemical signatures like alkali content.^[18] These limitations were highlighted in the 2002 IUGS revisions, which emphasized challenges with altered or fine-grained samples and recommended provisional classifications (e.g., using "-oid" suffixes) when modes are unavailable.^[7] No major structural updates to the QAPF scheme have occurred since the 2002 edition, though a 2024 preliminary IUGS report proposes minor refinements, such as replacing "M" with "X" for non-QAPF minerals to reduce ambiguity and excluding non-igneous minerals like scapolite from the plagioclase apex.^[4] In modern contexts, the diagram's dependence on traditional modal data can be outdated for digital analyses, such as automated petrographic imaging, where chemical or normative approaches may be more feasible without direct mineral quantification; emerging AI-enhanced methods for rock classification, including image-based mineral identification, highlight potential gaps in integrating QAPF with computational workflows as of 2025.^[21] To address these constraints, the IUGS recommends complementary methods for comprehensive classification. The Total Alkali-Silica (TAS) diagram serves as a primary chemical alternative, particularly for mafic and volcanic rocks where modal data is absent or unreliable, plotting total alkalis (Na₂O + K₂O) against silica (SiO₂) content in fresh samples (H₂O⁺ < 2%, CO₂ < 0.5%).^[7] For instance, the IUGS specifies QAPF for holocrystalline rocks with available modes and greater than 10% felsic minerals (Q + A + P + F > 10%), but defaults to TAS otherwise, especially for basalts and andesites.^[7] Harker diagrams complement QAPF by visualizing chemical trends across magmatic series, such as variations in oxides (e.g., SiO₂ vs. CaO) to identify differentiation paths in plutonic suites. The CIPW norm calculation provides normative mineral estimates from whole-rock chemistry, approximating modes for fine-grained or altered rocks unsuitable for direct QAPF plotting, such as aphyric volcanics. Hybrid approaches, combining QAPF with TAS or normative data, enable robust identification while mitigating over-reliance on visual modal assessment alone; for example, initial QAPF placement can be verified against TAS fields to reconcile modal and chemical discrepancies in volcanic sequences.^[7] This integrated strategy aligns with IUGS guidelines for plutonic rocks, where QAPF remains central but is supplemented for chemical context in series analysis.^[4]