Zr garnet from intrusive carbonatites of Kerimasi volcano, Tanzania

Zaitsev A.N.*, Williams C.T.**, Kuznetsova I.V.***, Britvin S.N.*, Spratt J**., Petrov S.V.*

* St. Petersburg State University, St. Petersburg, Russia; ** The Natural History Museum, London, UK;

*** St. Petersburg University of Technology and Design, St. Petersburg, Russia.

Kerimasi is an extinct carbonatite-nephelinite volcano located in the Gregory rift in northern Tanzania. It is composed of nephelinitic pyroclastic rocks with carbonatite agglomerates and tuffs (Mariano, Roeder 1983, Hay 1983, Church 1995).

Carbonatites here are represented by both intrusive and extrusive types. They occur as blocks up to 0.5 m in diameter. Intrusive carbonatites are calcitic in composition and contain forsterite pseudomorphs, magnetite and apatite as minor mineral phases and pyrochlore, baddeleyite and calzirtite as accessory minerals.

Zr-rich garnet was found in some samples of calcite carbonatites as well as in eluvial deposits on the eastern slope of the volcano and from explosive craters of Loluni, Kisete and Lulmurwak. The Zr-rich garnet occurs as spherical crystals up to 40 mm in carbonatite blocks and as euhedral to subhedral crystals up to 80 mm in carbonatite eluvium (Fig. 1). Microscopically the mineral is light to dark-brown, transparent with vitreous luster. It is isotropic with n = 1.945(5).

Microprobe analyses of the mineral show some variations in chemical composition and it contains 32.6-36.6 wt.% ZrO₂, 26.6-28.0 CaO, 15.0-17.8 Fe₂O₃, 7.4-10.0 SiO₂ and 3.1-7.4 Al₂O₃. The mineral is also Nb-rich with Nb₂O₅ ranging from 2.1 to 4.8 wt.%.

The average empirical formula of the mineral is

(Ca_2.996Mn_0.010)_S3.006(Zr_1.719Nb_0.139Ti_0.078Mg_0.024Y_0.016Ce_0.012Nd_0.009La_0.003)_S2.000(Fe³⁺_1.234Si_0.857Al_0.825Ti_0.084)_S3.000O_12.000

The crystal structure of the mineral is very similar to the structure of Zr garnet kimzeyite with the ideal formula Ca₂Zr₂(Al₂Si)O₁₂ (Milton et al. 1961) or Ca₂(Zr,Ti)₂(Si,Al,Fe³⁺)O₁₂ (http://pubsites.uws.edu.au/ima-cnmnc/) (Munno et al. 1980, Schingaro et al. 2001). It is cubic, space group Ia3d and a = 12.549(1) Е.

The mineral can be considered as the ferrite-dominant member of zirconium garnet series with ideal formula Ca₂Zr₂(Fe³⁺₂Si)O₁₂.

References:

Church A.A. The petrology of the Kerimasi carbonatitevolcano and the carbonatites of Oldoinyo Lengai with a review of other occurrences of extrusive carbonatites // PhD thesis. 1996. London: University of London. 384 p.

Hay R.L. Natrocarbonatite tephra of Kerimasi volcano, Tanzania // Geology. 1983. Vol. 11. P. 599-602.

Mariano A.N., Roeder P.L. Kerimasi: a neglected carbonatite volcano // J. Geology. 1983. Vol. 91. P. 449-455.

Milton C., Ingram B.L., Blade L.V. Kimzeyite, a zirconium garnet from magnet Cove, Arkansas // American Mineralogist. 1961. Vol. 46. P. 533-548.

Munno R., Rossi G., Tadini C. Crystal chemistry of kimzeyite from Stromboli, Aeolian Islands, Italy // American Mineralogist. 1080. Vol. 65. P. 188-191.

Schingaro E., Scordari F., Capitanio F., Parodi G., Smith D.C., Mottana A. Crystal chemistry of kimzeyite from Anguillara, Mts. Sabatini, Italy // European Journal of Mineralogy. 2001. Vol. 13. P. 749-759.