Paratooite-(La), a new lanthanum-dominant rare-earth copper carbonate from Paratoo, South Australia

摘要

Paratooite-(La) is a new lanthanum-dominant rare-earth copper carbonate from the Paratoo copper mine, near Yunta, Olary district, South Australia. Paratooite-(La) occurs as sheaves and radiating sprays of blade-like to tabular pale blue crystals in thin fissures in a slaty country rock. Individual crystals are typically 50-200 mu n in maximum dimension but < 5 mu m thick. Associated minerals include donnayite-(Y), kamphaugite-(Y), and bastnasite-(La). Electron microprobe and CHN analyses gave: La_2O_3 26.47; Pr_2O_3 7.74; Nd_2O_3 8.15; Sm_2O_3 0.66; Gd_2O_3 0.85; Y_2O_3 0.72; CaO 7.57; SrO 3.15; Na_2O 3.3; CuO 5.77; F 0.24; CO_2 32.05; NO_2 1.12; -O=F -0.10; sum 100.03, yielding an empirical formula of (La_(3.54)Ca_(2.94)Na_(2.32)Nd_(1.05)Pr_(1.03)Sr_(o.66)Y_(0.14)Gd_(0.10)Sm_(0.08))_(SIGMA 11.86)Cu_(1.58)(C_(15.84)N_(0.53))O_(47.76)F_(o 24) The simplified formula is (REE,Ca,Na,Sr)_6Cu(CO_3)_8 or possibly REE_3(Ca,Sr)_2NaCu(CO_3)_8. The mineral is pale turquoise-blue to pale blue in colour, transparent, with a pearly to vitreous lustre and a pale blue streak. No cleavage was observed but the morphology and TEM studies indicate a cleavage parallel to {100}. The Mohs hardness is estimated to be 4. The strongest lines in the X-ray powder pattern are [d_(obs) (I_(obs)) (hkl)]: 5.047 (53) (200); 4.786 (49) (021); 3.957 (43) (220); 3.468 (43) (012, 221); 2.927 (100) (202); 2.530 (52) (241); 2.344 (22) (420,103); 2.232 (20) (421). A synchrotron powder diffraction pattern was indexed on a primitive orthorhombic cell with a = 10.0862(5), b = 12.8088(6), c = 7.2360(4) A, V= 934.8(1) A~3 and Z= 2. The crystal structure of the new mineral could not be determined but powder diffraction data indicate the space group is probably P222, Pmmm, P222_1 or Pmm2. The measured density is 3.68(3) g/cm~3 and the calculated density is 3.78 g/cm~3. Paratooite-(La) is biaxial negative with a = 1.605(3), beta = 1.696(3) and gamma = 1.752(2); pleochroism is medium strong; X very pale bluish, Y and Z bluish (with greenish tint) with absorption Z approx = Y X. Paratooite-(La) is a supergene mineral which precipitated from mildly basic carbonated groundwaters. The mineral is named for the type locality.