Arisite-(La), a new REE-fluorcarbonate mineral from the Aris phonolite (Namibia), with descriptions of the crystal structures of arisite-(La) and arisite-(Ce) P. C. PIILONEN 1, *, A. M. MCDONALD 2 , J. D. GRICE 1 , M. A. COOPER 3 , U. KOLITSCH 4 , R. ROWE 1 , R. A. GAULT 1 AND G. POIRIER 1 1 Research Division, Canadian Museum of Nature, Ottawa, Ontario K1P 6P, Canada 2 Department of Earth Sciences, 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 NaLa 2 (CO 3 ) 2 [F 2x (CO 3 ) 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 temperature and sinks in methylene iodide, D calc. = 4.072 g cm À3 . 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%, wR 2 = 4.31% (Ce) and R = 2.09%, wR 2 = 5.26% (La), respectively. Arisite is hexagonal, P6 ¯ m2, Z = 1, with unit-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 have crystal structures characterized by separate layers of triangular planar CO 3 2À groups that parallel the overall layering of the structure, F, REE and alkali or alkaline-earth elements. Overall, the arisite structure can be defined by three distinct layers which parallel (001): (1) ?[REE(CO 3 ) 2 F] slabs, (2) sheets of Naf 9 polyhedra, and (3) ?[2F/CO 3 ] 2À . 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, NaY 2 (CO 3 ) 3 F. KEYWORDS: fluorcarbonate, REE, Aris phonolite, structure determination, new mineral species. Introduction THERE are 25 known fluorcarbonate mineral species, 19 of which contain essential concentra- tions of rare-earth elements (REE) in their structure. The majority are the product of late- stage, often hydrothermal, crystallization in alka- line rock complexes including carbonatites, phonolites, nepheline syenites and their associated pegmatites, with a minor presence in granitic rocks. They are commonly associated with other carbonates, bicarbonates or REE-carbonates, fluorides, phosphates and silicates. Rare-earth * E-mail: ppiilonen@mus-nature.ca DOI: 10.1180/minmag.2010.074.2.257 Mineralogical Magazine, April 2010, Vol. 74(2), pp. 257–268 # 2010 The Mineralogical Society