Zajacite-(Ce) is a new mineral that occurs as anhedral grains, 1-2 mm across, sparingly disseminated in hypersolvus granite at the Strange Lake Zr-Y-REE-Nb-Be deposit, on the Quebec-Labrador boundary northeast of Schefferville, Quebec. The new mineral is colorless to pale pink, has a vitreous luster, white streak, conchoidal fracture, H (Mohs)3 1/2, no cleavage, D(meas.) 4.44(1) g/cm 3 , and is uniaxial positive, with indices of refraction of omega 1.483(1), epsilon 1.503(1). Zajacite-(Ce) is a fluoride; electron-microprobe analyses of seven grains gave a mean of Na 6.4, Ca 11.5, Y 2.2, La 11.3, Ce 22.0, Nd 8.0, Sm 1.1, Gd 2.9, Dy 0.4, F 35.4, total 101.2 wt%, corresponding to Na (sub 0.90) [(REE) (sub 1.12) Ca (sub 0.92) ] (sub Sigma 2.04) F 6 , wherein Ce is the predominant REE. Single-crystal X-ray study showed the mineral to be trigonal, space group P3, a 6.099(1), c. 11.064(2) Aa, as refined from a Gandolfi pattern, with strongest lines [d in Aa(l)(hkl)] at 5.29(70)(100), 3.036(100)(110,103), 2.146(70)(203), 1.757(80)(300, 213), 1.152 (40)(410), and 0.9189(40)(513). The new name honors geologist Ihor Stephan Zajac, who led the exploration group that discovered the Strange Lake deposit, and who first recognized the presence of the new mineral.

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