Crystal structures and isotypism of the iron(III) arsenate kamarizaite and the iron(III) phosphate tinticite

Author(s)
Uwe Kolitsch, Christian Lengauer, Gerald Giester
Abstract

The crystal structures of the secondary ferric iron minerals kamarizaite, Fe

3

3+(AsO

4)

2(OH)

3 · 3H

2O, and tinticite, Fe

3

3+(PO

4)

2(OH)

3 · 3H

2O, for which highly contradictory data on crystal symmetry were reported, were studied by a combination of single-crystal X-ray diffraction and Rietveld refinement (supplemented by chemical analyses and thermogravimetry), using type material of both species and additional samples from several other localities, including the type localities. The previously unknown crystal structure of kamarizaite was determined from single-crystal intensity data (MoKα, 293 K, R(F) = 2.91 %; all H atoms detected) using a sample from the Le Mazet vein, Échassières, Auvergne, France. The mineral is triclinic, space group P1 (no. 2), with a = 7.671(2), b = 8.040(2), c = 10.180(2) Å, α = 68.31(3), β = 75.35(3), γ = 63.52(3)°, V = 519.3(2) $A$R

3, Z = 2. Rietveld analyses of fine-grained kamarizaite collected underground at two different spots in Lavrion, Greece (Hilarion and Jean Baptiste areas) confirmed the structure model. Rietveld analyses of fine-grained tinticite from Tintic, Utah (USA), Bruguers (Spain) and Weckersdorf (Germany) demonstrate that kamarizaite and tinticite are triclinic and isotypic. A previously published structure model for tinticite, as well as the originally reported orthorhombic symmetry for kamarizaite, are shown to be incorrect. Refined unit-cell parameters of acotype tinticite specimen from Tintic are: a = 7.647(1), b = 7.958(1), c = 9.987(1) Å, α = 67.90(1), β = 76.10(1), γ = 64.10(1)°, V = 504.4(2) Å. Bruguers and Weckersdorf tinticite have very similar parameters. The common atomic arrangement is characterised by three unique, octahedrally coordinated Fe sites (on which Fe may be partially replaced by minor Al), two unique tetrahedrally coordinated T (As or P) sites, eight O, three O

h, three O

w and nine H sites. The topology features zig-zag chains along [110] of dimers built of two edge-sharing FeO

6 octahedra corner-linked by a third FeO

6 octahedron. The chains are corner-linked by the TO

4 tetrahedra thus establishing a mixed octahedral-tetrahedral framework with a T:Fe ratio of 0.67, a pronounced layered arrangement parallel to (001) and narrow channels along [010]. Medium-strong to weak hydrogen-bonds provide additional strengthening of the structure. The topology is closely related to that of the recently described, triclinic aluminium phosphate afmite.

Organisation(s)
Department of Mineralogy and Crystallography
External organisation(s)
Naturhistorisches Museum Wien (NHM)
Journal
European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
Volume
28
Pages
71-81
No. of pages
11
ISSN
0935-1221
DOI
https://doi.org/10.1127/ejm/2015/0027-2485
Publication date
03-2016
Peer reviewed
Yes
Austrian Fields of Science 2012
105116 Mineralogy, 105113 Crystallography
Keywords
ASJC Scopus subject areas
Geochemistry and Petrology
Portal url
https://ucrisportal.univie.ac.at/en/publications/ec16a477-ba15-400c-a2d6-41bdd4bcb33f