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Arisite-(La), ideally NaLa2(CO3)2[F2x(CO3)1-x]F, is a new layered AEE-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, beigeyellow, 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- 31/2, is not fluorescent in either long- or shortwave UV radiation, dissolves slowly in dilute HC1 at room temperature and sinks in methylene iodide, Dcalc.= 4.072 g cm-3. Arisite-(La) is uniaxial negative, has sharp extinction, with both OD and 8 exhibiting a range of values within each grain: = 1.696-1.717(4) and = 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%o(La), respectively. Arisite is hexagonal, P6m2, Z=1, with unit-cell parameters of a = 5.1109(2) A, c = 8.6713(4) A, V = 196.16(6) A3for arisite-(Ce), and a = 5.1131(7) A, c = 8.6759(17) A, V= 196.43(5) A3for arisite-(La). Arisite-(Ce) and arisite-(La) are members of the layered, flat-lying AEEE-fluorcarbonate group which have crystal structures characterized by separate layers of triangular planar CO32-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 Na9polyhedra, 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. 2010 The Mineralogical Society.

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Compilation and indexing terms, Copyright 2018 Elsevier Inc.<br/>20102413012032<br/>Alkaline-earth elements<br/>Aris phonolite<br/>Fluorcarbonate<br/>Methylene iodide<br/>Miarolitic cavity<br/>New mineral species<br/>Structure determination<br/>Unit cell parameters