In:
Mineralogical Magazine, Mineralogical Society, Vol. 79, No. 2 ( 2015-04), p. 269-283
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).
Type of Medium:
Online Resource
ISSN:
0026-461X
,
1471-8022
DOI:
10.1180/minmag.2015.079.2.06
Language:
English
Publisher:
Mineralogical Society
Publication Date:
2015
detail.hit.zdb_id:
2034522-7
SSG:
13
Permalink