Fundamental Minerals

Sialic Minerals

Feldspar

Feldspathoid

Quartz

Femic Minerals

Accessory Minerals

Very Rare Minerals

Secondary Minerals

Texture

Alteration Products

Glass and related Texture

Volcanic Rocks

Komatiites

Lamprophyres

Natrocarbonatite

The natrocarbonatite lavas consist of the water-soluble phenocryst minerals nyerereite, (Na,K)₂Ca(CO₃)₂, and gregoryite, (Na,Ca,K)₂CO₃, in a matrix of symplectically intergrown fluorite and (halite-sylvite). Apatite, magnetite, monticellite, cuspidine-niocalite, sellaite, alabandite, sphalerite and galena occur as trace minerals.

Continuous lava effusion from Oldoinyo Lengai since 1988 has provided the opportunity for sampling and testing all of the stages of atmospheric alteration and hydration of natrocarbonatites. Identification of pre-1917 carbonatites in the former, pre-1917, crater platform and in the overflow fields of the steep north flank extended the observational base considerably. Most of these transformed older natrocarbonatites, form loose, sandy masses, but may contain more solid relict cores. Field observations indicate three major processes that lead to the formation of subsolidus (secondary) minerals at Oldoinyo Lengai (i) by sublimation from hot gases escaping during lava cooling, (ii) by atmospheric alteration and reaction with meteoric water and (iii) by reaction with fumarole gases. Subsolidus mineralogical re-equilibration of the lava starts immediately after solidification and cooling, transforming them, under atmospheric influences (air and meteoric water), in a very short time into white secondary products. Immediately after solidification, the natrocarbonatite lavas are cut by a network of thin cracks. The edges of the cracks become covered by white, brownish and yellowish encrustations up to several centimeters in thickness which are known as "efflorescence" or "salt fringes and tubes".

The origin of natrocarbonatite

Natrocarbonatite flows have emerged periodically over the last 4 decades in the summit crater of the Oldoinyo Lengai, accompanied by H₂O-CO₂ vapor. The currently favored explanation for the genesis of these carbonatites by liquid immiscibility between a silicate and a carbonatite melt is questioned based on the extremely low eruption temperatures of 544-593°C and compositional and mineralogical characteristics not in agreement with experimental constraints. Experimental investigations of the relationship between Oldoinyo Lengai natrocarbonatite and related silicate rock compositions do indicate that alkali-bearing peralkaline carbonatite with liquidus calcite can form by liquid immiscibility. At the same time, these experiments result in evidence which speaks against a liquid immiscibility origin for the highly alkaline and peralkaline Oldoinyo Lengai natrocarbonatite. No natural silicate magma is known to produce natrocarbonatite compositions by liquid immiscibility. The best interpretation of the Oldoinyo Lengai natrocarbonatite involves expulsion of a cognate, mobile, alkaline, and CO₂-rich fluid condensate.

The activity at Oldoinyo Lengai can be divided into three main types: (1) dormant stage; (2) carbonatite stage, and (3) Plinian stage. Dormant stages are periods with no activity or only minor degassing, carbonatite stages encompass periods of carbonatite extrusion and enhanced degassing, while Plinian stages refer to silicate dominated eruptions during which the silicate cone of the volcano is constructed. The dormant stage
During dormant periods (Fig.3) fluid is suggested to separate at depth from cooling low-alkali carbonatite, or carbonated peralkaline silicate melt in the subvolcanic magma chamber of the volcano. The fluid will react with previously crystallized phases (auto-metasomatism) and precipitate solids and/or leave droplets of carbonatite liquid behind in the fenitized zone above the magma chamber. This would be due to a decrease in solubility of the natrocarbonatite component with falling P and T, and equilibration with the mineral assemblages of the host silicate rocks (evolved nephelinite and phonolite). Volatiles dominated by CO₂ and H₂O will penetrate and react with the host rocks and be lost.

Fig.3: Dormant stage. After Nielsen, T. F (2002).

Carbonate stage
During moderately active silicate magmatism e.g., replenished of the magma chamber, previously deposited natrocarbonatite liquid and solid components could be reactivated to form small melts pools. The natrocarbonatite melt may contain variable amounts of silicate phase as supported by the presence of silicate melt droplets in some natrocarbonatite lavas. The natrocarbonatite liquid accumulates in small pockets, less than 10 m in radius, in the upper part of the volcanic edifice (Fig.4); natrocarbonatite and accompanying vapor are extruded from these little chamber and continued loss of CO₂ and H₂O vapor transform the fluid into natrocarbonatite melt. The resulting natrocarbonatite liquid will be dry and have strongly enhanced alkali content.

Fig.4: Carbonate stage. After Nielsen, T. F (2002).

Plinian stage
The magma chamber is replenished from the feeder system and the residing magma is expelled during violent Plinian eruption (Fig.5). Carbonatite melt in the small pockets or in the conduit may be entrained. After a Plinian eruption the volcano would return to a carbonatite or dormant stage.

Fig.5: Plinian stage. After Nielsen, T. F (2002).

View of Lengai from Engaro Sero village. From Photovolcanica.

Oldoinyo Lengai Crater, July 2004. From Photovolcanica.

Hornitos on crater floor seen from E rim, July 2004. Dark lava flow is only hours old. From Photovolcanica.

Hornitos on crater floor seen from E rim, July 2004. Dark lava flow is only hours old. From Photovolcanica.

Nighttime lava flow from hornito in July 2004. From Photovolcanica.

Lava flow from vent in flank of hornito, July 2004. Dark lava flow is only hours old. From Photovolcanica.

Lava cascading down flank of hornito, July 2004. Dark lava flow is only hours old. From Photovolcanica.

Fumarolic deposits formed on fresh lava within 24 hours. Dark lava flow is only hours old. From Photovolcanica.

Fumarolic deposits formed on fresh lava within 24 hours. Dark lava flow is only hours old. From Photovolcanica.

Fumarolic deposits formed on fresh lava within 24 hours. Dark lava flow is only hours old. From Photovolcanica.

Bibliography

• A. N. Zaitsev; J. Keller (2006): Mineralogical and chemical transformation of Oldoinyo Lengai natrocarbonatites, Tanzania. Lithos 91 (2006) 191-207.
• A. Simonetti; K.Bell; C. Shrady (1997): Trace and rare earth element geochemistry of the June 1993 natrocarbonatite la vas, Oldoinyo Lengai (Tanzania): Implications for the origin of carbonatite magmas. Journal of Volcanology and Geothermal Research 75 (1997) 89-106.
• J. Klaudius; J. Keller (2006): Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania. Lithos 91 (2006) 173-190.
• Keller, J., Zaitsev, A. N., & Wiedenmann, D. (2006). Primary magmas at Oldoinyo Lengai: the role of olivine melilitites. Lithos, 91(1), 150-172.
• Kervyn, M., Ernst, G. G. J., Klaudius, J., Keller, J., Mbede, E., & Jacobs, P. (2008). Remote sensing study of sector collapses and debris avalanche deposits at Oldoinyo Lengai and Kerimasi volcanoes, Tanzania. International Journal of Remote Sensing, 29(22), 6565-6595.
• Nielsen, T. F., & Veksler, I. V. (2002). Is natrocarbonatite a cognate fluid condensate? Contributions to Mineralogy and Petrology, 142(4), 425-435.
• Roger H. Mitchell (2009): Peralkaline nephelinite–natrocarbonatite immiscibility and carbonatite assimilation at Oldoinyo Lengai, Tanzania. Contrib Mineral Petrol (2009) 158:589-598.
• Vaughan, R. G., Kervyn, M., Realmuto, V., Abrams, M., & Hook, S. J. (2008). Satellite measurements of recent volcanic activity at Oldoinyo Lengai, Tanzania. Journal of Volcanology and Geothermal Research, 173(3), 196-206.

Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)
Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Rounded Gregorite crystals and tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 10x (Field of view = 2mm)	Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 10x (Field of view = 2mm)	Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 10x (Field of view = 2mm)
Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)
Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. PPL image , 2x (Field of view = 7mm)	Tabular Nyerereite crystals and Rounded Gregorite crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 2x (Field of view = 7mm)
Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Tabular Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)	Nyerereite (altered) crystals in a Natrocarbonatite from Ol Doinyo Lengai. XPL image , 10x (Field of view = 2mm)