Almeidaite, Pb(Mn,Y)Zn (sub 2) (Ti,Fe (super 3+) ) (sub 18) O (sub 36) (O,OH) (sub 2) ; a new crichtonite-group mineral, from Novo Horizonte, Bahia, Brazil

Almeidaite (IMA 2013-020), ideally Pb(Mn,Y)Zn (sub 2) (Ti,Fe (super 3+) ) (sub 18) O (sub 36) (O,OH) (sub 2) , from Novo Horizonte, Bahia, Brazil, occurs in association with quartz, rutile, anatase, hematite, kaolinite, muscovite, xenotime-(Y) and bastnaesite-(La). Almeidaite forms isolated, black, opaque, sub-metallic, platy crystals flattened on [0001], measuring up to 30 mm X 30 mm X 6 mm in size, dominated by the basal pinacoid {0001}, which is bounded by various, mostly steep, rhombohedra and the hexagonal prism {112I0}. Most of the crystals are multiply twinned, with non-planar contact surfaces that are approximately parallel to the c axis. The streak is brown. Reflectance values are [(R (sub o) , R (sub e) ) lambda (nm)]: (12.78, 15.39) 470; (12.86, 15.43) 546; (12.91, 15.55) 589; (13.04, 15.75) 650. The empirical formula is (Pb (sub 0.59) Sr (sub 0.12) Ca (sub 0.04) La (sub 0.03) ) (sub Sigma 0.78) (Mn (sub 0.54) Y (sub 0.46) ) (sub Sigma 1.00) Zn (sub 1.43) (Ti (sub 13.02) [Fe (super 3+) (sub 4.98) ) (sub Sigma 18.00) (Fe (super 3+) (sub 0.32) Mn (sub 0.15) ) (sub Sigma 0.47) [O (sub 37.18) (OH) (sub 0.82) ] (sub Sigma 38.00) . It is trigonal, space group R3I, with the unit-cell parameters a = 10.4359(2), c = 21.0471(4) Aa, V = 1985.10(7) Aa (super 3) and Z = 3. The crystal structure was solved (R (sub 1) = 0.039) using 2110 unique reflections with I > 3sigma (I). Almeidaite is a member of the crichtonite group with Pb dominant in the A site (with 12-fold coordination) and Zn dominant in the T site (with 4-fold coordination). It is a Zn analogue of senaite and a Pb analogue of landauite. The mineral is named after Professor Fernando Flavio Marques de Almeida (1916-2013).