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

2015; Cambridge University Press; Volume: 79; Issue: 2 Linguagem: Inglês

10.1180/minmag.2015.079.2.06

1471-8022

Luiz Alberto Dias Menezes Filho, Н. В. Чуканов, Р. К. Расцветаева, Sergey M. Aksenov, Igor V. Pekov, Mario Luiz de Sá Carneiro Chaves, R. Peter Richards, Daniel Atencio, Paulo Roberto Gomes Brandão, Ricardo Scholz, Klaus Krambrock, Roberto L. Moreira, Frederico Sousa Guimarães, Antônio Wilson Romano, Aba C. Persiano, Luiz C.A. Oliveira, José D. Ardisson,

Advanced Condensed Matter Physics

Abstract Almeidaite (IMA 2013-020), ideally Pb(Mn, Y)Zn2(Ti,Fe 3+ ) 18 O 36 (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 6 30 mm 6 6 mm in size, dominated by the basal pinacoid {0001}, which is bounded by various, mostly steep, rhombohedra and the hexagonal prism {1120}. 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 o , R e ) 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 (Pb 0.59 Sr 0.12 Ca 0.04 La 0.03 ) Σ0.78 (Mn 0.54 Y 0.46 ) Σ1.00 Zn 1.43 (Ti 13.02 Fe 4.98 3+ ) Σ18.00 (Fe 0.32 3+ Mn 0.15 ) Σ0.47 [O 37.18 (OH) 0.82 ] Σ38.00 . It is trigonal, space group R 3, with the unit-cell parameters a = 10.4359(2), c = 21.0471(4) Å, V = 1985.10(7) Å 3 and Z = 3. The crystal structure was solved ( R 1 = 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 Flávio Marques de Almeida (1916–2013).