REDEFINITION OF A MINERAL IN THE JOAQUINITE GROUP: ORTHOJOAQUINITE-(La)

Orthojoaquinite-(La) from Ilimaussaq, Greenland, is a new member of the joaquinite group. Its description was published by Semenov et al. (1967); although the chemical analytical data indicated that La is the dominant REE in the mineral, its true position in the group was not established for a third of a century. Orthojoaquinite-(La) is orthorhombic, probable space-group Ccmm, a 10.539(10), b 9.680(5), c 22.345(10) A, V 2280 A 3 , Z = 4. Chemical analytical data are: Na 2O 2.41, K2O 0.22, CaO (+SrO) 0.03, MnO 0.70, FeO 4.78, BaO 21.46, Fe2O3 0.39, La2O3 10.05, Ce2O3 9.40, Pr2O3 0.99, Nd2O3 2.15, SiO2 33.82, TiO2 9.20, ThO2 0.38, Nb2O5 2.31, H2O 1.50, F 0.38, sum 100.17, less O = F 0.16, total 100.01 wt.%. The empirical formula (based on O + OH + F + H2O = 28) is: (Ba1.99Ca0.01)� 2.00(Na1.11K0.07)� 1.18(La0.88Ce0.81Nd0.18Pr0.09)� 1.96 (Fe 2+ 0.95Mn0.14)� 1.09 (Ti1.64Nb0.25Fe 3+ 0.07 Th0.02)� 1.98 Si8.01O26.00[(OH)0.37O0.35F0.28]� 1.00•1.00H2O. The simplified formula is Ba2Na(La,Ce)2Fe 2+ Ti2Si8O26(OH,O,F)•H 2O. The mineral is brown, has a silky luster, and is transparent. The Mohs hardness is about 5 (VHN 350–430 kg/mm 2 ); it has a good {001} cleavage. The measured density is 4.1 g/cm 3 , and the calculated density is 4.14 g/cm 3 . Optical data: optically biaxial (+), � 1.754, � 1.760, � 1.797; 2V(meas.) 40°, 2V(calc.) 45°; orientation Z = c; pleochroism strong in yellow-green, Z > X.