| definition |
Fine-grained or porphyritic igneous rock with less than 20 percent quartz, and less
than 10 percent feldspathoid minerals, in which the ratio of plagioclase to total
feldspar is greater 0.65. Typically composed of calcic plagioclase and clinopyroxene;
phenocrysts typically include one or more of calcic plagioclase, clinopyroxene, orthopyroxene,
and olivine. Includes rocks defined modally in QAPF fields 9 and 10 or chemically
in TAS field B as basalt. Basalt and andesite are distinguished chemically based on
silica content, with basalt defined to contain less than 52 weight percent silica.
If chemical data are not available, the color index is used to distinguish the categories,
with basalt defined to contain greater than 35 percent mafic minerals by volume or
greater than 40 percent mafic minerals by weight.
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