Return to Alkaline Rocks

Alkaline rocks contain a variety of pyroxene-group minerals, but only aegirine (NaFe3+Si2O6) is exclusive to these rocks. Sodic pyroxenes found in alkaline rocks and carbonatites are typically members of a complex solid solution involving aegirine, diopside (CaMgSi2O6) and hedenbergite (CaFe2+Si2O6), but some also feature high levels of Ti, Al, Mn and Zr. Compositionally intermediate pyroxenes are sometimes grouped under the name aegirine-augite. Large crystals of aegirine-augite and aegirine are black, whereas fibrous, felty and radiating aggregates of small slender crystals are green. Thanks to their crystal shape, color and 90o-cleavage, sodic pyroxenes are readily recognizable under the polarizing microscope (see here). Shown on this page are:
[Upper right] Spherulites of aegirine in calcite carbonatite from the Murun complex in Siberia. [Lower right] Phenocryst of aegirine-augite in phonolitic nephelinite from the Kerimasi volcano in Tanzania, as seen in back-scattered electrons under the scanning-electron microscope. Note its beautiful zoning pattern arising from several growth "spurts" interrupted by reaction with the host magma and phenocryst resorption. The lighter-gray zones are richer in Na and Fe, but poorer in Ca and Mg relative to the darker-gray zones. The black hexagonal crystals in the groundmass are nepheline.
Aegirine spherulites in calcite carbonatite

Aegirine-augite phenocryst in phonolitic nephelinite