• journal_title：Mineralogical Magazine
• Contributor：P. C. Piilonen ; A. M. McDonald ; J. D. Grice ; M. A. Cooper ; U. Kolitsch ; R. Rowe ; R. A. Gault ; G. Poirier
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1180/minmag.2010.074.2.257
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：74
• issue：2
• firstpage：257
• section：Research Articles

d="p-1">Arisite-(La), ideally NaLa₂(CO₃)₂[F_2x(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%, wR² = 4.31% (Ce) and R = 2.09%, wR² = 5.26% (La), respectively. Arisite is hexagonal, Pclass="inline-formula" id="inline-formula-1">class="math tex" alt="Formula" src="257/embed/tex-math-1.gif" />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 class="inline-formula" id="inline-formula-2">class="math tex" alt="Formula" src="257/embed/tex-math-2.gif" /> 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.