Author:
Menezes Filho Luiz A. D.,Chukanov Nikita V.,Rastsvetaeva Ramiza K.,Aksenov Sergey M.,Pekov Igor V.,Chaves Mário L. S. C.,Richards R. Peter,Atencio Daniel,Brandão Paulo R. G.,Scholz Ricardo,Krambrock Klaus,Moreira Roberto L.,Guimarães Frederico S.,Romano Antônio W.,Persiano Aba C.,de Oliveira Luiz C. A.,Ardisson José D.
Abstract
AbstractAlmeidaite (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 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 [(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(Ti13.02Fe4.983+)Σ18.00(Fe0.323+Mn0.15)Σ0.47[O37.18(OH)0.82]Σ38.00.
It is trigonal, space group R3, 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 Flávio Marques de Almeida (1916–2013).
Subject
Geochemistry and Petrology
Cited by
11 articles.
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