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 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
(Pb0.59Sr0.12Ca0.04La0.03)Σ0.78(Mn0.54Y0.46)Σ1.00Zn1.43(Ti13.02Fe4.98
3+)Σ18.00(Fe0.32
3+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 (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).