phenocryst

From Mindat.Org Photo Of The Day; March 1, 2015:

Augite (Var: Titanian Augite): (Ca,Na)(Mg,Ti, Fe,Al,)(Si,Al)2O6

Locality: Limberg Quarries, Sasbach, Kaiserstuhl, Baden-Württemberg, Germany

An interpenetrant twin of titanian augite, which also displays sector compositional zoning and fine oscillatory zoning (thin section, plane polarized light). The version with crossed polars (child photo)shows the variations of crystallographic orientation and composition very clearly. The crystal group is present as a phenocryst in low-silica, high Ti+alkalies lava termed limburgite (from the location, not the cheese). Igneous petrology has since moved away from such localized terms: the lava would now probably be called basanite, which broadly speaking is intermediate in composition between alkali basalt and nephelinite. The Kaiserstuhl is an important nephelinite-carbonatite volcanic center. Photo by Tony Peterson