Arisite-(La), a new REE-fluorcarbonate mineral from the Aris phonolite (Namibia), with descriptions of the crystal structures of arisite-(La) and arisite-(Ce)

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 3½, 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−3. Arisite-(La) is uniaxial negative, has sharp extinction, with both ω and ϵ exhibiting a range of values within each grain: ω = 1.696–1.717(4) and ϵ = 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%, w R 2 = 4.31% (Ce) and R = 2.09%, w R 2 = 5.26% (La), respectively. Arisite is hexagonal, P ![Formula][1] m 2, 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 ![Formula][2] 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 (CO3)2F] slabs, (2) sheets of Naφ9 polyhedra, and (3) ∞[2F/CO3]2−. 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. [1]: /embed/tex-math-1.gif [2]: /embed/tex-math-2.gif