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Carlhintzeite, Ca2AlF7·H2O, from the Gigante granitic pegmatite, Córdoba province, Argentina: description and crystal structure

Published online by Cambridge University Press:  05 July 2018

A. R. Kampf ,
F. Colombo  and
J. González del Tánago
A. R. Kampf*
Affiliation:
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Blvd, Los Angeles California 90007, USA
F. Colombo
Affiliation:
CICTERRA-CONICET - Cátedra de Geología General, Facultad de Ciencias Exactas, Físicas y Naturales – Pabellón Geología, Universidad Nacional de Córdoba, Vélez Sarsfield 1611, (X5016GCA) Córdoba, Argentina
J. González del Tánago
Affiliation:
Departamento de Petrología y Geoquímica, Facultad de Ciencias Geológicas, Universidad Complutense, 28040 Madrid, Spain
*
*E-mail: akampf@nhm.org
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Abstract

Carlhintzeite, Ca2AlF7·H2O, has been found at the Gigante pegmatite, Punilla Department, Córdoba Province, Argentina. It occurs as colourless prismatic crystals up to 0.8 mm long, ubiquitously twinned on {001}. Electron microprobe analyses provided the empirical formula Ca1.98Al1.02F6.24(OH)0.76·H1.62O. A crystal fragment used for the collection of structure data provided the triclinic, C1̄ cell: a = 9.4227(4), b = 6.9670(5), c = 9.2671(7) Å, α = 90.974(6), β = 104.802(5), γ = 90.026(6)°, V = 558.08(7) Å3 and Z = 4. The crystal structure, solved by direct methods and refined to R1 = 0.0322 for 723 Fo > 4σF reflections, is made up of linkages of AlF6 octahedra, CaF8 polyhedra and CaF6(H2O)2 polyhedra. The AlF6 octahedra are isolated from one another, but share polyhedral elements with Ca polyhedra. Most notably, the Al1 octahedron shares trans faces with two CaF8 polyhedra and the Al2 octahedron shares trans edges with two CaF6(H2O)2 polyhedra. The linkage of the Ca polyhedra alone can be described as a framework in which edge-sharing chains along b are cross-linked by edge-sharing. Edge-sharing chains of Ca polyhedra along b in the carlhintzeite structure are similar to those along c in the structures of gearksutite, CaAlF4(OH)·(H2O), and prosopite, CaAl2F4(OH)4.

Keywords

carlhintzeitegearksutiteprosopitecrystal structureCórdobaArgentina
Type
Research Article
Information
Mineralogical Magazine , Volume 74 , Issue 4 , August 2010 , pp. 623 - 632
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2010

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