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A New REE-Fluorcarbonate Mineral from the Aris Phonolite (Namibia), with Descriptions of the Crystal Structures of Arisite-(La) and Arisite-(Ce)

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A New REE-Fluorcarbonate Mineral from the Aris Phonolite (Namibia), with Descriptions of the Crystal Structures of Arisite-(La) and Arisite-(Ce) Mineralogical Magazine, April 2010, Vol. 74(2), pp. 257–268 Arisite-(La), a new REE-fluorcarbonate mineral from the Aris phonolite (Namibia), with descriptions of the crystal structures of arisite-(La) and arisite-(Ce) 1, 2 1 3 4 1 1 P. C. PIILONEN *, A. M. MCDONALD ,J.D.GRICE ,M.A.COOPER ,U.KOLITSCH ,R.ROWE ,R.A.GAULT 1 AND G. POIRIER 1 ResearchDivision, Canadian Museum of Nature, Ottawa, Ontario K1P 6P, Canada 2 Department of EarthSciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada 3 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada 4 Mineralogisch-Petrographische Abt., Naturhistorisches Museum, Burgring 7, A-1010 Wien, Austria [Received 14 January 2010; Accepted 14 April 2010] ABSTRACT Arisite-(La), ideally NaLa2(CO3)2[F2x(CO3)1Àx]F, is a new layered REE-fluorcarbonate mineral from miarolitic cavities within the Aris phonolite, Namibia (IMA no. 2009-019). It occurs as distinct chemical zones mixed with its Ce-analogue, arisite-(Ce). Crystals are vitreous, transparent beige, beige- yellow, light lemon-yellow to pinkish, and occur as tabular prisms up to 1.5 mm. Arisite-(La) is brittle, has conchoidal fracture, poor cleavage perpendicular to (001), a Mohs hardness of ~3À3Ý, is not fluorescent in either long- or shortwave UV radiation, dissolves slowly in dilute HCl at room À3 temperature and sinks in methylene iodide, Dcalc. = 4.072 g cm . Arisite-(La) is uniaxial negative, has sharp extinction, with both o and e exhibiting a range of values within each grain: o = 1.696À1.717(4) and e = 1.594À1.611(3), a result of chemical zoning attributed to both Ce > La and Na > Ca substitutions. The crystal structure of both arisite-(Ce) and arisite-(La) were solved by direct methods and refined to R = 1.66%, wR2 = 4.31% (Ce) and R = 2.09%, wR2 = 5.26% (La), respectively. Arisite is hexagonal, P6¯m2, Z = 1, withunit-cell parameters of a = 5.1109(2) A˚ , c = 8.6713(4) A˚ , V = 196.16(6) A˚ 3 for arisite-(Ce), and a = 5.1131(7) A˚ , c = 8.6759(17) A˚ , V = 196.43(5) A˚ 3 for arisite-(La). Arisite-(Ce) and arisite-(La) are members of the layered, flat-lying REE-fluorcarbonate group which 2À have crystal structures characterized by separate layers of triangular planar CO3 groups that parallel the overall layering of the structure, F, REE and alkali or alkaline-earthelements. Overall, thearisite structure can be defined by three distinct layers which parallel (001): (1) ?[REE(CO3)2F] slabs, 2À (2) sheets of Naf9 polyhedra, and (3) ?[2F/CO3] . Based on its (M+F)/C ratio, arisite can further be described as having a dense, flat-lying fluorcarbonate structure, a classification which includes the structurally related mineral species cordylite, kukharenkoite, cebaite, lukechangite, huanghoite, and one incompletely characterized synthetic phase, NaY2(CO3)3F. KEYWORDS: fluorcarbonate, REE, Aris phonolite, structure determination, new mineral species. Introduction tions of rare-earthelements ( REE)intheir THERE are 25 known fluorcarbonate mineral structure. The majority are the product of late- species, 19 of which contain essential concentra- stage, often hydrothermal, crystallization in alka- line rock complexes including carbonatites, phonolites, nepheline syenites and their associated pegmatites, witha minor presence in granitic rocks. They are commonly associated with other * E-mail: [email protected] carbonates, bicarbonates or REE-carbonates, DOI: 10.1180/minmag.2010.074.2.257 fluorides, phosphates and silicates. Rare-earth # 2010 The Mineralogical Society P. C. PIILONEN ET AL. fluorcarbonate minerals, in particular bastna¨site, breccias. The phonolite and trachyte rocks occur provide the majority of the world’s supply of as dykes and plugs which are exposed as eroded REE. One major deposit, the Bayan Obo outcrops and caps on elevated hills and buttes. carbonatite in Inner Mongolia, north-central The Aris phonolite dyke occurs in the southern China, provides much of the world’s require- part of the province and is currently being mined ments. Rare-earthfluorcarbonate minerals havea for road and building material at the Ariskop and growing economic potential as their unusual Railroad quarries. The phonolite is fine- to optical and magnetic properties are important medium-grained, withan aphyrictexture, and is for industrial applications suchas catalysis, composed predominantly of sanidine, nepheline permanent magnets, glass and ceramic manufac- and aegirine, withaccessory hau ¨yne, leucite, ture, phosphors, lasers, bubble magnetic monazite and zircon (von Knorring and Franke, memories, and solid-oxide fuel-cell (SOFC) 1987). It also contains numerous miarolitic electrodes and electrolytes. As a result, synthesis cavities that range from 0.1 mm to 10 cm in of REE-fluorcarbonate species has become diameter, many of which are ‘wet’, containing increasingly important, bothfor industry and for residual hydrothermal formational fluids which providing suitable material for mineralogical are released when the cavities are broken open. studies of natural mineral species which do not Preliminary X-ray computed tomography at the yield suitable crystals for crystal-structure University of Texas (Austin) indicates that analysis (Mercier and Leblanc, 1993a,b,c; Grice 15À20% of the phonolite is comprised of et al., 2007). Many of these synthetic compounds miarolitic cavities. In some samples, 80% of have natural equivalents, yet many do not. these cavities are two-phase fluid + vapour-filled Synthesis studies allow us to investigate further vugs. The miarolitic cavities host a range of the conditions under which these unusual agpaitic mineral species, including abundant minerals crystallize. villiaumite, aegirine, labuntsovite-group minerals, Arisite-(La), and its Ce-analogue, arisite-(Ce) tuperssuatsiaite, natrolite, analcime, mangano- (Piilonen et al., 2010) are two naturally occurring neptunite, apophyllite-(KF), fluorite, and maka- mineral species which, as of yet, do not have a tite. The presence of ubiquitous villiaumite in the stable synthetic equivalent. Arisite-(La) was cavities suggests a H2O-poor environment. An discovered in phonolite from the Aris quarry, extensive list of the known minerals within the Namibia, and is named for its type locality. Both Aris quarries has been given by Sturla et al. the mineral and the name have been approved by (2005). A detailed study of the mineralogy and the IMA CNMMN (IMA no. 2009-019). Type paragenesis of these miarolitic cavities is on- material has been deposited at the Canadian going. Museum of Nature, Ottawa (CMNMC 86076). Physical and optical properties Occurrence Arisite-(La) is visually indistinguishable from its The Aris phonolite in Namibia is part of the late Ce analogue, arisite-(Ce) (Piilonen et al., 2010). It Tertiary (33Ô1 Ma, Fitchand Miller, 1984) Auas is brittle, has conchoidal fracture, poor cleavage alkaline volcanic province, which extends 65 km parallel to (001), a Mohs hardness of ~3À3Ý, is from Windhoek in the north to Rehoboth in the non-fluorescent under either long- or short-wave south. This volcanic province represents the most UV radiation, and dissolves slowly withefferves- recent occurrence of alkaline magmatism along cence in dilute HCl at room temperature. Arisite- the western margins of southern Africa, similar to (La) sinks in methylene iodide (i.e. D À3 À3 the Klinghardt Mountain alkaline volcanic >3.3 g cm ) and has a Dcalc. of 4.072 g cm , province further to the south (37 Ma, Kro¨ner, Z = 1. Arisite-(La) occurs as euhedral, hexagonal 1973; Marsh, 1987). The Auas alkaline volcanic plates up to 1.5 mm (average: 0.2 mm 6 1.0 mm) province intrudes metasedimentary rocks (quartz- and rare tabular, hexagonal prisms, in miarolitic feldspar gneiss, mica schist and amphibolite) of cavities. The crystals are vitreous, transparent, the Paleoproterozoic Hohewarte complex. Gevers and range in colour from beige, beige-yellow, (1934) identified over 100 occurrences of both light lemon-yellow to light pink. There is often a intrusive and extrusive alkaline rocks including heavily included zone between the core and rim trachyte, phonolite, shonkinite, alkali peridotite, which imparts a clouded appearance to the and also a variety of tuffs, agglomerates and crystals. Crystals are zoned, witharisite-(La) 258 STRUCTURE DETERMINATION OF ARISITE-(LA) AND ARISITE-(CE) cores and arisite-(Ce) rims, withadditional minor suitability of the fluorite standard. Count times for Ca > Na substitution between core and rim. all elements were 25 s, or shorter if a precision of Chemical zoning is gradational from core to rim, 0.5% had already been attained, with 25 s count withinfrequent patchy,mottled zones. Not all times for the background. Table 1 contains crystals at Aris are zoned, and arisite-(Ce) is the EMPA data for arisite-(La) and arisite-(Ce) from dominant species. Observed forms include a Aris, Namibia. dominant {001} pinacoid, witha minor {100} In order to determine the carbon content of prism. Re-entrant angles have been noted on arisite, crystals were analysed by laser-ablation thicker crystals, but the twin law is not known. inductively coupled plasma mass spectrometry Arisite-(La) is uniaxial negative, has sharp (LA-ICP-MS). Two arisite grains were analysed extinction, withboth o and e exhibiting a range for 13C (at.%) using a beam size of 80 mm, 60% of values within each grain: o = 1.696À1.717(4) power, 20.8 J cmÀ2 fluence and a 5 Hz laser and e = 1.594À1.611(3), a result of chemical pulse. A standard of natural bastna¨site-(Ce) zoning attributed to bothCe > La and Na > Ca (Madagascar) was also employed. The average substitutions. Associated minerals include number of counts (n = 5) obtained from the NAM aegirine, analcime, apatite, fluorite, mangano- crystals was 3895 sÀ1 and for the bastna¨site-(Ce) neptunite, microcline, natrolite, sphalerite, tupers- (n = 2), 4417 sÀ1. This suggests that the total C suatsiaite, the unnamed, Fe-analogue of concentration is less than that of bastna¨site-(Ce) zakharovite, and arisite-(Ce).
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