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(named for the village of Tholey, in the Saar, Federal Republic of Germany), a volcanic rock devoid of porphyritic inclusions. Tholeiite is composed of basic plagioclase (labradorite), pyroxenes (pigeonite, augite, hypersthene), basaltic hornblende, quartz, and sometimes olivine. The interstices between the components contain particles of unaltered volcanic glass, with microlites and dendrites of plagioclase and ore mineral. The entire mix is known as a tholeiitic structure. Upon decomposition of the glass, tholeiite is converted into quartz-potassium-feldspar granophyres characteristic of traprock. Tholeiite is the most widely distributed of the petrochemical basalts, which are slightly supersaturated with silica; it is contrasted with alkaline (olivine) basalt, which is incompletely saturated with SiO2.
The study of tholeiite is of significant theoretical value in solving problems related to magma formation, since tholeiite reflects the composition of the upper mantle in regions where magmatism has occurred.