View metadata, citation and similar papers at core.ac.uk brought to you by CORE Published in "Mineralogical Magazine 73(3): 373-384, 2009" which should be cited to refer to this work. provided by RERO DOC Digital Library 2+ AlumoÔkermanite, (Ca,Na)2(Al,Mg,Fe )(Si2O7), a new mineral from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, northern Tanzania 1,2, 3,{ 4 4 5 D. WIEDENMANN *, A. N. ZAITSEV ,S.N.BRITVIN ,S.V.KRIVOVICHEV AND J. KELLER 1 Department of Geoscience, University of Fribourg, Perolles, 1700 Fribourg, Switzerland 2 EMPA, Swiss Federal Laboratories for Material Testing and Research, Hydrogen and Energy, Ueberlandstrasse 129, Duebendorf 8600, Switzerland 3 Department of Mineralogy, Faculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia 4 Department of Crystallography, Faculty of Geology, St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg 199034, Russia 5 Institut fu¨r Geowissenschaften, Mineralogie-Geochemie, Universita¨t Freiburg, Albertstrasse 23b, 79104 Freiburg, Germany ABSTRACT 2+ Alumoa˚kermanite, (CaNa)2(Al,Mg,Fe )(Si2O7), is a new mineral member of the melilite group from the active carbonatite-nephelinite-phonolite volcano Oldoinyo Lengai, Tanzania. The mineral occurs as tabular phenocrysts and microphenocrysts in melilite-nephelinitic ashes and lapilli-tuffs. Alumoa˚ker- manite is light brown in colour; it is transparent, with a vitreous lustre and the streak is white. Cleavages or partings are not observed. The mineral is brittle with an uneven fracture. The measured density is 2.96(2) g/cm3. The Mohs hardness is ~4.5À6. Alumoa˚kermanite is uniaxial (À) with o = 1.635(1) and e = 1.624À1.626(1). In a 30 mm thin section (+N), the mineral has a yellow to orange interference colour, straight extinction and positive elongation, and is nonpleochroic. The average chemical formula of the mineral derived from electron microprobe analyses is: (Ca1.48Na0.50Sr0.02 2+ 3+ K0.01)S2.01(Al0.44Mg0.30Fe0.17Fe0.07Mn0.01)S0.99(Si1.99Al0.01O7). Alumoa˚kermanite is tetragonal, space ˚ ˚ ˚ 3 group P4¯21m with a = 7.7661(4) A, c = 5.0297(4) A, V = 303.4(1) A and Z = 2. The five strongest ˚ powder-diffraction lines [d in A,(I/Io), hkl] are: 3.712, (13), (111); 3.075, (25), (201); 2.859, (100), http://doc.rero.ch (211); 2.456, (32), (311); 1.757, (19), (312). Single-crystal structure refinement (R1 = 0.018) revealed structure topology typical of the melilite-group minerals, i.e. tetrahedral [(Al,Mg)(Si2O7)] sheets interleaved with layers of (CaNa) cations. The name reflects the chemical composition of the mineral. KEYWORDS: alumoa˚kermanite, a˚kermanite, melilite, new mineral, volcanic rock, chemical composition, crystal structure, Oldoinyo Lengai, Tanzania. Introduction natrocarbonatites (e.g., Keller and Krafft, 1990). Nevertheless, the main part of the about 3000 m OLDOINYO LENGAI, located in the Gregory Rift in high cone consists of silicate lavas and pyroclasts the north of Tanzania, is the only active (Donaldson et al., 1987; Klaudius and Keller, carbonatite volcano in the world. The volcano is 2006). Beside phonolites and nephelinites, primi- famous for its effusive and highly viscous tive olivine-melilities and evolved combeite/ melilite-bearing nephelinites play an important role in the evolution of Oldoinyo Lengai. * E-mail: [email protected] Melilite occurs in different volcanic rocks at DOI: 10.1180/minmag.2009.073.3.373 and around Oldoinyo Lengai. It is a major mineral 1 in olivine melilitites from the minor volcanic Names of the IMA, proposal 2008-049. The centres Dorobo cone, Lalarasi, Kirurum and cotype specimens are deposited in the Armycon Hill (Dawson et al., 1985; Keller et Mineralogical Museum, Department of al., 2006). Melilite in nephelinitic volcanics was Mineralogy, St. Petersburg State University, St. reported from the parasitic Nasira cones (Dawson, Petersburg, Russia (sample OL 218, catalogue 1989), from a lava of the 1917 paroxysmal number 1/19407) and the Fersman Mineralogical eruption from the main Lengai crater (Keller Museum, Russian Academy of Sciences, and Krafft, 1990), but also from an alkalic Moscow, Russia (sample OL 244, catalogue pyroxenite block from yellow Lengai I phonolitic number 3823/1). pyroclasts (Donaldson and Dawson, 1978). Additionally, melilite was described from silicate globules in natrocarbonatite from the 1993 Mineral occurrence eruption (Dawson et al., 1996; Petibon et al., Alumoa˚kermanite was found in a number of 1998) and carbonate-silicate lapilli and ashes of volcanic rocks during a systematic mineralogical both the 1966 and 2007 eruptions (Dawson et al., study of melilite from the Oldoinyo Lengai 1989, 1992; Mitchell and Dawson, 2007). volcano and surrounding area (Wiedenmann, The melilite from the olivine melilitites exhibits 2004). The new mineral occurs in evolved a relatively narrow range in composition. It is olivine-free melilite-nephelinitic ashes and a˚kermanitic with large MgO (8.6À9.9 wt.%), lapilli-tuffs of the 1966/67 and pre-1966 explo- subordinate Al2O3 (3.9À5.9 wt.%) and small sive eruptions, in particular, the 1940/1941 ashes, Na2 O(2.3À3.3 wt.%) and FeOtotal together with nepheline, aegirine-augite but (2.7À3.7 wt.%). A˚ kermanite with large MgO and seldom with combeite. Microcrysts of the matrix small Na2OandAl2O3 contents (7.0À10.1, are nepheline, aegirine-augite, combeite, spinel 1.3À3.6 and 0À4.2 wt.% respectively) occurs group minerals and sporadic wollastonite, mela- also in carbonate-silicate lapilli and ashes of the nite, titanite and sodalite. 1966 eruption, but it is an Fe-rich variety with In addition, melilite with a composition 6.7À9.3 wt.% FeOtotal. Melilite from nephelinites, intermediate between that of a˚kermanite and carbonate-silicate lapilli of the 2007 eruption and alumoa˚kermanite has been studied. The melilite some lapilli of the 1966 eruption, as well as occurs as idiomorphic crystals up to 1.5 cm size in silicate globules in natrocarbonatite and alkalic unconsolidated ashes within the stratigraphic pyroxenite, forms a distinct compositional group higher levels of the volcano. These recent ashes À the mineral is a low-Mg variety (3.5À6.0 wt.% and lapilli are classified as combeite-wollastonite- MgO), but is enriched in Na2O (4.9À6.0 wt.%), melilite nephelinites. Al2 O 3 (5.1À8.5 wt.%) and FeOtotal (5.5À10.9 wt.%). Enrichment of this melilite in Physical properties http://doc.rero.ch Al is so great that some of the analyses show dominance of Al over Mg in the T1 site, e.g. Alumoa˚kermanite occurs as tabular phenocrysts 2+ 3+ (Ca1.52Na0.49)S2.01(Al0.43Mg0.34Fe0.16Fe0.06)S0.99 (up to 1.5 mm) and microphenocrysts in a fine- (Si1.99Al0.01O7) (analysis 3, table 5 in Donaldson grained groundmass (Fig. 1). Macroscopically, and Dawson, 1978) and (Ca1.48Na0.53K0.01)S2.02 the mineral is light brown in colour; it is 2+ 3+ (Al0.42Mg0.25Fe0.20Fe0.10)S0.97(Si2.01O 7) transparent with a vitreous lustre, and the streak (analysis 8, table 8 in Dawson, 1998). is white. Cleavages or partings are not observed. Unfortunately, no other data have been reported It is brittle with an uneven fracture. Long- or for this Na-Al-rich melilite. short-wave ultraviolet radiation produces no In this paper we describe a new mineral from observable fluorescence. The measured density, the Oldoinyo Lengai volcano which is a member determined by sink-float in heavy liquids for of the melilite group. The new mineral has been sample OL 244, is 2.96(2) g/cm3, close to the named alumoa˚kermanite and this name reflects its value of 3.00 g/cm3 calculated from average chemical composition: Ca is the dominant cation empirical formulae and refined cell dimensions in the X site. Aluminium is absent in the T2 site for sample OL 244. The Mohs hardness for (a˚kermanite) and the dominance of Al over Mg in OL 244 is ~4.5À5. the T1 site is indicated by the prefix ‘alumo’. Both Alumoa˚kermanite is uniaxial (À) with o = the mineral and mineral name have been approved 1.635(1), e = 1.626(1) for sample OL 244 and o = by the Commission on New Mineral and Minerals 1.635(1), e = 1.624(2) for sample OL 218 (Na 2 FIG. 1. Tabular crystals of alumoa˚kermanite (yellow to yellow-orange and orange-red) in nephelinitic ash. Other minerals are nepheline (grey À from dark to light), wollastonite and diopside. Crossed nicols, field of view is 160.74 mm. Sample OL 244. light, 589 nm). In thin section, under cross- melilite, while complex intergrowths lead to polarized light, alumoa˚kermanite has a yellow to spherical or star-like shapes (Fig. 2). A micro- orange-red interference colour; it has straight indentation (microhardness, VHN load 25 g), test extinction and positive elongation, and is non- produced a mean = 700 g/mm2, in the range pleochroic. 635À755 g/mm2 (10 measurements were made on Melilite of intermediate composition between a polished block by C. Stanley, The Natural a˚kermanite and alumoa˚kermanite (samples OL History Museum, London) which is equivalent to mel1 and OL mel2) occurs as idiomorphic a Mohs hardness of about 5.5À6 for OL mel1 and crystals up to 1.5 cm size (Fig. 2). Melilite is OL mel2. brown to dark brown in colour, semi-transparent and with a vitreous lustre. The crystals are short, prismatic and tabular with well-developed square Chemical composition {001} faces. The crystals are commonly inter- Wavelength-dispersive microprobe analyses of grown, both simply and in a complex fashion. alumoa˚kermanite were obtained using a Cameca Simple intergrowths preserve the tabular habit of SX100 electron microprobe operated at 15 kV, http://doc.rero.ch FIG.