![]() QAPF diagrams are also not used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites).Īn exact name can be given only if the mineralogical composition is known, which cannot be determined in the field. TAS is also used if volcanic rock contains volcanic glass (such as obsidian). 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. ![]() QAPF diagrams are mostly used to classify plutonic rocks ( phaneritic rocks), but are also used to classify volcanic rocks if modal mineralogical compositions have been determined. Geologists worldwide accept the diagrams as a classification of igneous, especially plutonic rocks. QAPF diagrams were created by the International Union of Geological Sciences (IUGS): Subcommission on the Systematics of Igneous Rocks įostered by Albert Streckeisen (whence their alternative name: Streckeisen diagrams).
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