http://www.cnr.it/ontology/cnr/individuo/prodotto/ID29139
The crystal-chemistry of holmquistites: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles. (Articolo in rivista)
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- The crystal-chemistry of holmquistites: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles. (Articolo in rivista) (literal)
- Anno
- 2005-01-01T00:00:00+01:00 (literal)
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- Cámara F.; Oberti R. (literal)
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- FERNANDO CÁMARA* AND ROBERTA OBERTI
CNR-Istituto di Geoscienze e Georisorse, via Ferrata 1, I-27100 Pavia, Italy (literal)
- Titolo
- The crystal-chemistry of holmquistites: Ferroholmquistite from Greenbushes (Western Australia) and hints for compositional constraints in BLi amphiboles. (literal)
- Abstract
- determine
the reasons for their limited compositional variations. Structural constraints to the relative stability
of BLi amphiboles are also suggested by the occurrence of ferro- and ferri-ferroclinoholmquistites, and
the lack of clinoholmquistite. Detailed crystal-chemical analysis shows: (1) a remarkable constancy in
composition, both in terms of charge arrangement and of limited homovalent M1,3(Mg.1Fe2+), M2(Al.1Fe3+),
and O3(OH.1F) exchanges, (2) a remarkable constancy in the unit-cell dimensions, with the Fe3+ content
at the M2 site being the only factor affecting the b edge; (3) complete ordering of Li at the M4 site,
in contrast with the common partitioning between the M4 and M3 sites in clinoamphiboles, which
however couples with partial A-site occupancy; (4) complete ordering of trivalent cations at the M2
site; (5) an inverse relationship between the Fe2+ and the Fe3+ contents, which is interpreted as a way
to keep the size of the octahedral strip constant; (6) a strong distortion of the octahedral sites, both in
terms of angular variance and quadratic elongation.
A¦BLi2
C(Mg3Al2)TSi8O22(OH)2 is the amphibole composition composed of the smallest possible
structural moduli. Crystallization in Pnma symmetry, where the two double-chains of tetrahedra can
assume different conformations, is probably required by the need to obtain a more suitable [5 + 1]-
coordination for BLi, and to shrink the cation-cation distances. This arrangement does not allow for
extensive incorporation of larger homovalent substituents, which are hosted via mechanisms implying
distortion of the octahedral sites.
During this work, a sample with Fe2+ slightly but signiÞ cantly higher than Mg was characterized, and
then recognized as a mineral species by the IMA-CNMMN (2004-030). Holotype ferroholmquistite has
a = 18.287 (1), b = 17.680 (1), and c = 5.278 (1) Å, and V = 1706.6(1) Å3. Its crystal-chemical formula
is AK0.01Na0.01
B(Li1.88Mg0.08Na0.03Fe2+
0.01)C(Al1.89Fe2+
1.70Mg1.39Mn2+
0.02)TSi8.00O22(OH1.97F0.03). Ferroholmquistite
occurs as elongated black to bluish-violet prismatic crystals; it is biaxial negative, with a = 1.628, ß
= 1.646, and . = 1.651 (. = 589 nm), 2Vx (calc.) = 55.1°. It is weakly pleochroic, with a = colorless,
ß = pale violet-blue, and . = blue to deep violet; the calculated density is 3.145 g/cm3. The holotype
specimen belongs to the mineral collection of Renato and Adriana Pagano (Italy), and comes from the
Greenbushes pegmatite (Western Australia). The analyzed sample has been deposited at the Museum
of the Dipartimento di Scienze della Terra, Università di Pavia (Italy) under the code 2004-01. (literal)
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