Cathodoluminescence Observations of Natrocarbonatites and Related Peralkaline Nephelinites at Oldoinyo Lengai

Cathodoluminescence (CL) is utilized for the identification and characterization of mineral phases crystallizing from natrocarbonatites of the recent eruptions of Oldoinyo Lengai. The Nacarbonates, nyerereite and gregoryite, show dominantly Mn-activated orange luminescence, quite similar to calcites and dolomites of carbonatites. Nyerereite from older carbonatites is distinguished by atypical blue luminescence. CL also reveals the presence and textural arrangement of submicroscopic sylvite, fluorite and apatite. Among the alteration products in natrocarbonatites, nahcolite is characterized by green CL colours whereas pirssonite and gaylussite luminesce red and blue.Zoning in nepheline, wollastonite, combeite and apatite from a combeite-nephelinite lava is documented. The CL spectroscopy shows that Mn2+ is the main activator in wollastonite, combeite, and in the green cores of apatite. The spectral analysis of blue apatite indicates REE activation, considered typical of apatites from carbonatites.