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Mineralogical Magazine; October 2006; v. 70; no. 5; p. 565-578; DOI: 10.1180/0026461067050350
© 2006 Mineralogical Society of Great Britain and Ireland
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Crystal-structure refinement of a Zn-rich kupletskite from Mont Saint-Hilaire, Quebec, with contributions to the geochemistry of zinc in peralkaline environments

P. C. Piilonen1, I. V. Pekov2, M. Back3, T. Steede3 and R. A. Gault4

1 Earth Sciences Division, Canadian Museum of Nature, Ottawa, Ontario K1P 6P4, Canada
2 Faculty of Geology, Moscow State University, Borobievy Gory, 119992 Moscow, Russia
3 Natural History Department, Mineralogy, Royal Ontario Museum, Toronto, Ontario M5S 2C6, Canada
4 Earth Sciences Division, Canadian Museum of Nature, Ottawa, Ontario K1P 6P4, Canada

* E-mail: ppiilonen{at}mus-nature.ca

The chemistry and crystal structure of a unique Zn-rich kupletskite: (K1. 55 Na 0 .2 1 Rb 0. 09 Sr 0 . 01 ){sum}1 .8 6 (Na 0 .8 2 Ca 0 . 18 ) {sum}1 .0 0 (Mn 4. 7 2 Zn 1 .6 6 Na 0. 41 Mg 0. 12 Fe0. 092+) {sum}7 . 00 (Ti1.85Nb0.11Hf0.03){sum}1.99(Si7.99Al0.12){sum}8.11O26 (OH)4(F0.77OH0.23){sum}1.00, from an alkaline pegmatite at Mont Saint-Hilaire, Quebec, Canada has been determined. Zn-rich kupletskite is triclinic, PFormula, a = 5.3765(4), b = 11.8893(11), c = 11.6997(10), {alpha} = 113.070(3), ß = 94.775(2), {gamma} = 103.089(3), R1 = 0.0570 for 3757 observed reflections with Fo > 4{sigma}(Fo). From the single-crystal X-ray diffraction refinement, it is clear that Zn2+ shows a preference for the smaller, trans M(4) site (69%), yet is distributed amongst all three octahedral sites coordinated by 4 O2– and 2 OH– [M(2) 58% and M(3) 60%]. Of note is the lack of Zn in M(1), the larger and least-distorted of the four crystallographic sites, with an asymmetric anionic arrangement of 5 O2– and 1 OH–. The preference of Zn for octahedral sites coordinated by mixed ligands (O and OH) is characteristic of its behaviour in alkaline systems, in contrast to granitic systems where Zn tends to favour [4]-coordinated, OH- and H2O-free sites with only one ligand species (O, S, Cl, B, I). In alkaline systems, [4]Zn is only present in early sphalerite or in late-stage zeolite-like minerals. The bulk of Zn in alkaline systems is present as discrete [6]Zn phases such as members of the astrophyllite, labuntsovite, milarite and nordite groups, a result of the formation of network-forming Zn(OH)42– complexes in the low-temperature, low-fS2, high-alkalinity and highly oxidizing systems.

KEYWORDS: zinc, ordering alkaline, astrophyllite-group, pegmatite, kupletskite






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