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.

You do not currently have access to this article.