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To use this classification method, the concentrations (the modes) of the four mineral groups must be determined or estimated, and then normalized to 100%. Thus, for a rock identified as having, say, 20% mica, 30% quartz (Q), 30% alkali feldspar (A), and 20% plagioclase (P), the mica is disregarded,
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This diagram makes no distinction between rock types at the same QAPF plot position and classification, but of different bulk chemical compositions with respect to other minerals such as olivine, pyroxenes, amphiboles or micas. For example, because non-Q, -A, -P and -F minerals are disregarded the
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Of these, the (again) normalised relative proportions of A and P are 37.5/62.5 = 60% and 25/62.5 = 40%. 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. For this
418:
Streckeisen, A. L., 1978. IUGS Subcommission on the
Systematics of Igneous Rocks. Classification and Nomenclature of Volcanic Rocks, Lamprophyres, Carbonatites and Melilite Rocks. Recommendations and Suggestions. Neues Jahrbuch für Mineralogie, Abhandlungen, Vol. 141,
202:, 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. (Other mineral groups may occur in samples, but they are disregarded in this classification method.)
422:
Le Maitre, R.W. 2002. Igneous Rocks: A Classification and
Glossary of Terms : Recommendations of International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press,
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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
73:
groups used for classification in a QAPF diagram. The percentages (ratios) of the Q, A, P and F groups are normalized, i.e., recalculated so that their sum is 100%.
269:(July 1974). "Classification and nomenclature of plutonic rocks recommendations of the IUGS subcommission on the systematics of Igneous Rocks".
93:(whence their alternative name: Streckeisen diagrams). Geologists worldwide use the diagrams in classifying igneous, especially plutonic rocks.
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And, a plutonic rock that contains no feldspathoids (F group), no alkali feldspar (A group), but contains plagioclase-feldspar (P group), many
226:(not labeled in a QAPF diagram), and few quartz grains (Q group)—is probably gabbro; (see right edge of the Streckeisen diagram, at side P).
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An exact name can be given only if the mineralogical composition is established, which cannot be determined in the field.
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The QAPF diagram presents for use the proportions (ratios) of four plutonic mineral(s) or mineral groups, which are:
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and the normalized ratios (proportions) of the Q, A, and P groups are calculated as 37.5%, 37.5% and 25% = 100%.
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i.e., (30 + 30 + 20= 80; then 30/80= 37.5%, 30/80= 37.5%, and 20/80= 25%; therefore 37.5 + 37.5 + 25= 100%).
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are the most important of groups that have separate classification diagrams; (see
Streckeisen diagram).
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137:(Total-Alkali-Silica) is used. TAS is also used if volcanic rock contains volcanic glass (such as
160:). Instead, an alternate triangle plot diagram is used; (see Streckeisen diagram, lower right.)
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Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks".
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394:(2nd ed.). Cambridge, UK: Cambridge University Press.
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or volcanic rocks if modal mineralogical compositions are
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The QAPF diagram is not used for all plutonic rocks; the
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Classification of
Igneous Rocks - IUGS Classification
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British
Geological Survey: Rock Classification Scheme
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87:Subcommission on the Systematics of Igneous Rocks
23:QAPF diagram for classification of plutonic rocks
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392:Principles of igneous and metamorphic petrology
366:"Rock Classification Scheme - Vol 1 - Igneous"
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150:make up more than 90% of the rock composition
390:Philpotts, Anthony R.; Ague, Jay J. (2009).
121:have been determined. But QAPF diagrams are
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101:QAPF diagrams are mostly used to classify
83:International Union of Geological Sciences
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16:Classification system for igneous rocks
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230:system does not distinguish between
109:rocks), and can be used to classify
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314:Journal of the Geological Society
81:QAPF diagrams are created by the
119:modal mineralogical compositions
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141:). QAPF diagrams are not used
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247:ultramafic plutonic rocks
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344:10.1144/gsjgs.148.5.0825
167:Reading the QAPF diagram
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69:", which are the four
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271:Geologische Rundschau
189:plagioclase feldspars
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33:doubled-triangle plot
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443:on 30 September 2011
326:1991JGSoc.148..825L
283:1974GeoRu..63..773S
267:Streckeisen, Albert
175:Streckeisen diagram
67:Feldspathoid (Foid)
291:10.1007/bf01820841
177:
135:TAS classification
91:Albert Streckeisen
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458:Igneous petrology
401:978-0-521-88006-0
127:pyroclastic rocks
53:QAPF stands for "
38:used to classify
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413:Further reading
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115:aphanitic rocks
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89:as fostered by
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59:Alkali feldspar
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320:(5): 825–833.
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277:(2): 773–786.
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200:triangle plots
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152:(for example:
131:not determined
111:volcanic rocks
103:plutonic rocks
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191:(P), and the
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133:. There the
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29:QAPF diagram
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240:anorthosite
158:pyroxenites
154:peridotites
63:Plagioclase
452:Categories
253:References
107:phaneritic
47:mineralogy
468:Petrology
330:CiteSeerX
299:130569261
224:pyroxenes
187:(A), the
183:(Q), the
40:intrusive
352:28548230
148:minerals
139:obsidian
123:not used
85:(IUGS):
322:Bibcode
279:Bibcode
236:diorite
71:mineral
51:acronym
36:diagram
423:236pp.
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238:, and
232:gabbro
181:quartz
77:Origin
55:Quartz
49:. The
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369:(PDF)
348:S2CID
295:S2CID
195:(F).
145:mafic
117:) if
97:Usage
31:is a
396:ISBN
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340:doi
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Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
