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
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