Abstract

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, Dcalc. = 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%, wR2 = 4.31% (Ce) and R = 2.09%, wR2 = 5.26% (La), respectively. Arisite is hexagonal, P

\({\bar{6}}\)
m2, Z = 1, with unit-cell parameters of a = 5.1109(2) Å, c = 8.6713(4) Å, V = 196.16(6) Å−3 for arisite-(Ce), and a = 5.1131(7) Å, c = 8.6759(17) Å, V = 196.43(5) Å−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
\(\mathbf{\mathrm{CO}_{3}^{2-}}\)
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.

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