Publication Date:
2015-07-03
Description:
Almeidaite (IMA 2013-020), ideally Pb(Mn,Y)Zn 2 (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 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 o , R e ) (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 ) 0.78 (Mn 0.54 Y 0.46 ) 1.00 Zn 1.43 (Ti 13.02 $${\mathrm{Fe}}_{4.98}^{3+}$$ ) 18.00 ( $${\mathrm{Fe}}_{0.32}^{3+}$$ Mn 0.15 ) 0.47 [O 37.18 (OH) 0.82 ] 38.00 . It is trigonal, space group R 3I, 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 〉 3( 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).
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
Permalink