An aphanitic (fine-grained) igneous rock is classified as trachybasalt when it has a silica content of about 49% and a total alkali metal oxide content of about 6%. This places trachybasalt in the S1 field of the TAS diagram. Trachybasalt is further divided into sodium-rich hawaiite and potassium-rich potassic trachybasalt, with wt%Na2O > K2O + 2 for hawaiite.[3][4][5] The intrusive equivalent of trachybasalt is monzonite.[6]
Trachybasalt is not defined on the QAPF diagram, which classifies crystalline igneous rock by its relative content of feldspars and quartz.[3][4][5] However, the U.S. Geological Survey defines trachybasalt as a mafic volcanic rock (composed of over 35% mafic minerals) in which the quartz-feldspar-feldspathoid fraction of the rock is less than 20% quartz and less than 10% feldspathoid, and in which plagioclase is between 65% and 90% of the total feldspar content.[7]
^ abPhilpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143. ISBN9780521880060.
^Allaby, Michael (2013). "trachybasalt". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN9780199653065.
^Orlando, Andrea D'Orazio; Armienti, Pietro; Borrini, Daniele (29 August 2008). "Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)". European Journal of Mineralogy. 20 (4): 653–664. Bibcode:2008EJMin..20..653O. doi:10.1127/0935-1221/2008/0020-1841.
^Goff, Fraser; Kelley, Shari A.; Goff, Cathy J.; McCraw, David J.; Osburn, G. Robert; Lawrence, John R.; Drakos, Paul G.; Skotnicki, Steven J. (2019). "Geologic map of the Mount Taylor volcano area, New Mexico". New Mexico Bureau of Geology and Mineral Resources Geologic Map. 80.
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