Alkali olivine basalt is silica-undersaturated, characterized by the absence of orthopyroxene,
absence of quartz, presence of olivine, and typically contains some feldspathoid mineral,
alkali feldspar or phlogopite in the groundmass. Feldspar phenocrysts typically are
labradorite to andesine in composition. Augite is rich in titanium compared to augite
in tholeiitic basalt. Alkali olivine basalt is relatively rich in sodium.
note
The definition of tholeiite and alkali basalt here are more prescriptive 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.
source
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.
