Almeidaite, Pb(Mn,Y)Zn2(Ti,Fe3+)18O36(O,OH)2; a new crichtonite-group mineral, from Novo Horizonte, Bahia, Brazil

• Published in: Mineralogical magazine 2015-04, Vol.79 (2), p.269-283
• Main Authors: Menezes Filho, Luiz A. D, Chukanov, Nikita V, Rastsvetaeva, Ramiza K, Aksenov, Sergey M, Pekov, Igor V, Chaves, Mario L. S. C, Richards, R. Peter, Atencio, Daniel, Brandao, Paulo R. G, Scholz, Ricardo, Krambrock, Klaus, Moreira, Roberto L, Guimaraes, Frederico S, Romano, Antonio W, Persiano, Aba C, de Oliveira, Luiz C. A, Ardisson, Jose D
• Format: Article
• Language: English
• Subjects: almeidaite; anatase; Bahia; Bahia Brazil; bastnaesite; Brazil; carbonates; chemical composition; clay minerals; coexisting minerals; crichtonite group; crystal chemistry; crystal form; crystal structure; fluorides; formula; framework silicates; halides; hematite; kaolinite; landauite; lattice parameters; mica group; Mineralogy; muscovite; new mineral; new minerals; nonsilicates; Novo Horizonte; Novo Horizonte Brazil; oxides; phosphates; quartz; reflectance; rutile; senaite; sheet silicates; silica minerals; silicates; South America; space groups; unit cell; X-ray diffraction data; xenotime
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/minmag.2015.079.2.06

Almeidaite (IMA 2013-020), ideally Pb(Mn,Y)Zn2(Ti,Fe3+)18O36(O,OH)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 × 30 mm × 6 mm in size, dominated by the basal pinacoid {0001}, which is bounded by various, mostly steep, rhombohedra and the hexagonal prism {112İ0}. 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 [(Ro, Re) λ (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 (Pb0.59Sr0.12Ca0.04La0.03)Σ0.78(Mn0.54Y0.46) Σ1.00Zn1.43(Ti13.02[Fe3+4.98 )Σ18.00(Fe3+0.32 Mn0.15)Σ0.47[O37.18(OH)0.82]Σ38.00. It is trigonal, space group R3İ, with the unit-cell parameters a = 10.4359(2), c = 21.0471(4) Å, V = 1985.10(7) Å3 and Z = 3. The crystal structure was solved (R1 = 0.039) using 2110 unique reflections with I > 3σ(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).