Mapiquiroite, (Sr,Pb)(U,Y)Fe2 (Ti,Fe3+)18O38, a new member of the crichtonite group from the Apuan Alps, Tuscany, Italy

Abstract

Mapiquiroite, (Sr,Pb)(U,Y)Fe 2 (Ti,Fe 3+ ) 18 O 38 , was identified as a new mineral from the baryte + pyrite + iron oxide ores of Buca della Vena (BdV) and Monte Arsiccio (MA) mines, Apuan Alps, Tuscany, Italy. At BdV, mapiquiroite occurs as complex rhombohedral or tabular pseudo-hexagonal crystals, up to 1 mm in size, black in colour, with a sub-metallic lustre, in quartz + ‘adularia’ + baryte veins embedded in schists, in association with allanite-(Ce), anatase, destinezite, gypsum, monazite-(Ce), pyrite, rutile, and ‘tourmaline’; at MA, mapiquiroite forms tabular pseudo-hexagonal crystals, up to 5 mm in size, in quartz + carbonate veins embedded in magnetite-rich dolostones, in association with baryte, boulangerite, derbylite, siderite, and sphalerite. Under the microscope, mapiquiroite is blackish in colour, weakly bireflectant, and non-pleochroic. Internal reflections are absent. Anisotropism is distinct, without characteristic rotation tints. Minimum and maximum reflectance data for COM wavelengths [λ (nm), R air (%); data for the specimens from BdV and MA, respectively] are: 471.1, 17.0/17.2 and 17.7/18.0; 548.3, 16.7/17.0 and 17.6/17.8; 586.6, 16.4/16.7 and 17.3/17.7; 652.3, 16.1/16.3 and 17.0/17.3. Vickers hardness is 750 kg · mm −2 and 782 kg·mm −2 for specimens from BdV and MA, respectively, corresponding to a Mohs’ hardness ~ 6. The chemical data point to the formulae (Sr 0.533 La 0.176 Pb 0.105 Na 0.030 Ca 0.026 ) ∑=0.870 (U 0.407 Ce 0.101 Y 0.205 Mn 0.072 ) ∑=0.785 Fe 3+ 2.000 (Ti 12.423 Fe 3+ 3.345 Cr 1.624 V 5+ 0.129 Al 0.032 Sn 0.013 Nb 0.007 ) ∑=17.573 O 38 (BdV) and (Sr 0.312 Pb 0.248 Na 0.019 Ca 0.014 La 0.009 ) ∑=0.602 (U 0.858 Y 0.070 Ce 0.021 Mn 0.005 ) ∑=0.954 (Fe 3+ 1.695 Zn 0.305 ) ∑=2.000 (Ti 12.070 Fe 3+ 4.987 V 5+ 0.372 Al 0.030 Nb 0.030 Cr 0.015 Sn 0.001 ) ∑=17.505 O 38 (MA). Mapiquiroite is trigonal, R 3, with a 10.3719(7), c 20.875(1) A, V 1944.8(2) A 3 and a 10.3854(3), c 20.8942(6) A, V 1951.7(1) A 3 for the specimens from BdV and MA, respectively. The crystal structures of mapiquiroite from these two occurrences were refined to R 1 = 0.028 and 0.032, respectively. Mapiquiroite is isostructural with other members of the crichtonite group. The name mapiquiroite honours four Italian mineral collectors, Riccardo Mazzanti, Luigi Pierotti, Ugo Quilici, and Moreno Romani for their contribution to the study of the mineralogy of baryte + pyrite + iron oxide deposits from the Apuan Alps.