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Arisite-(La), ideally NaLa2(CO3)2[F2x(CO3)1 x]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 ~3 3@, is not fluorescent in either long- or shortwave UV radiation, dissolves slowly in dilute HCl at room temperature and sinks in methylene iodide, Dcalc. = 4.072 g cm 3 . Arisite-(La) is uniaxial negative, has sharp extinction, with both o and e exhibiting a range of values within each grain: o = 1.696 1.717(4) and e = 1.594 1.611(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 have crystal structures characterized by separate layers of triangular planar CO3 2 groups that parallel the overall layering of the structure, F, REE and alkali or alkaline-earthelements. Overall, the arisite structure can be defined by three distinct layers which parallel (001): (1) ?[REE(CO3)2F] slabs, (2) sheets of Naf9 polyhedra, and (3) ?[2F/CO3] 2 . Based on its (M+F)/C ratio, arisite can further be<br/>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.