Abstract

Cordylite-(La), (Na,Ca)2Ba2(La3Sr)∑4(CO3)8F2, is a new mineral species (IMA2010–058) from the Biraya Fe–REE deposit, Irkutsk district, Russia. It occurs in direct association with barite, biraite-(Ce), niobium-rich chevkinite-(Ce), fergusonite-(Nd), ancylite-(Ce) and ancylite-(La), daqingshanite-(Ce) and daqingshanite-(La), bastnäsite-(Ce), hydroxylbastnäsite-(Ce), carbocernaite, monazite-(Ce), talc, humite, thorite, pyrite, and pyrrhotite in carbonate veins composed of aragonite, strontianite, calcite, dolomite, and cordylite-(Ce). Cordylite-(La) forms as colorless, honey-yellow, or pinkish yellow, irregularly-shaped, tabular, or short-prismatic hexagonal crystals up to about 3 mm across. The only forms observed are {101̅0} and {0001} and crystals have perfect cleavage on {0001}. Crystals have a greasy to vitreous luster, a brittle tenacity, a conchoidal to uneven fracture, and a hardness of 4 on the Mohs scale. Crystals are uniaxial (−), ɛ = 1.573–1.574, and ω = 1.749–1.751. The empirical formula for two different crystals gave: (Na1.24Ca0.78)∑2.02Ba2.10[(La1.54Ce1.31Nd0.17Pr0.04)∑3.06Sr0.89]∑3.95(C1.01O3)8(F1.78OH0.05)∑1.83 and(Na1.23Ca0.79)∑2.02Ba1.98[(La1.43Ce1.33Nd0.25Pr0.10)∑3.11Sr0.94]∑4.05(C1.01O3)8(F1.51OH0.38)∑1.89. The crystal structure, solved on the same two crystals (R1 = 0.0246 and 0.0205) gave the unit cell parameters in P63/mcc: a = 5.1182(5) and c = 23.1785(16) Å and V = 525.84(16) Å3 and a = 5.1196(3) and c = 23.1784(16) Å and V = 525.13(5) Å3 with Z = 1, and confirms that the mineral is isostructural with cordylite-(Ce). We also discuss new heterovalent substitution mechanisms found in the cordylite group minerals and the effect they have during the formation of crystals.

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