CORDYLITE-(La), A NEW MINERAL SPECIES IN FENITE FROM THE BIRAYA Fe–REE DEPOSIT, IRKUTSK, RUSSIA

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: (Na_1.24Ca_0.78)_∑2.02Ba_2.10[(La_1.54Ce_1.31Nd_0.17Pr_0.04)_∑3.06Sr_0.89]_∑3.95(C_1.01O₃)₈(F_1.78OH_0.05)_∑1.83 and(Na_1.23Ca_0.79)_∑2.02Ba_1.98[(La_1.43Ce_1.33Nd_0.25Pr_0.10)_∑3.11Sr_0.94]_∑4.05(C_1.01O₃)₈(F_1.51OH_0.38)_∑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) ų and a = 5.1196(3) and c = 23.1784(16) Å and V = 525.13(5) ų 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.