Tholeiitic basalt is defined here to contain 2 pyroxene phases and interstitial quartz
or tridymite or cristobalite in the groundmass. Pyroxene (augite and orthopyroxene
or pigeonite) and calcium-rich plagioclase are common phenocryst minerals. Olivine
may also be a phenocryst, and when present, may have rims of pigeonite. Only in tholeiitic
basalt is olivine in reaction relationship with melt. Interstitial siliceous residue
may be present, and is often glassy. Tholeiitic basalt is relatively poor in sodium.
This category includes most basalts of the ocean floor, most large oceanic islands,
and continental flood basalts such as the Columbia River Plateau.
Definition of tholeiite and alkali basalt here are more proscriptive than those found
in most reference authorities. This is to actually provide some descriptive criteria
to allow assignment of rocks on a hand sample basis to the tholeiite or alkali basalt
categories if detailed petrographic or chemical data are available.
http://en.wikipedia.org/wiki/Basalt; Carmichael, I.S. Turner, F.J., Verhoogen, John,
1974, Igneous petrology: New York, McGraw HIll Book Co., p.42-43.