Langhof, J., Holtstam, D. & Gustafsson, L.
Chiavennite and zoned genthelvite-helvite as late-stage minerals of the Proterozoic LCT pegmatites at Utö, Stockholm, Sweden

Keywords
Beryllium, boron, chiavennite, genthelvite, helvite, late-stage mineralization, granitic pegmatite, Utö, Sweden

Abstract
The Utö pegmatite system is host to a plethora of late-formed minerals, most of which have not been detected in previous studies of this classical locality. Among them are minerals that developed in cracks and druses, including a number of beryllosilicates, like the zeolite chiavennite, genthelvite-helvite, milarite and bavenite; Mn-rich minerals such as manganite, wickmanite and friedelite; and several sulphides. The parageneses and crystal chemistry of chiavennite and genthelvite-helvite are the focus of the present paper. Chiavennite was analysed using electron and ion-microprobe techniques, and was found to contain significant amounts of B substituting for Be: a structural formula for one sample is (Ca_0.93Na_0.07)(Mn_0.77Fe²⁺_0.10Ca_0.07Fe³⁺_0.06Mg_0.01) [Be_1.86B_0.15][Si_4.68Al_0.32]O_12.86(OH)_2.14 · 2H₂O. The corresponding orthorhombic unit-cell parameters are a = 8.907(4), b = 31.227(9), and c = 4.777(2) Å. The genthelvite-helvite grains, often arranged in polycrystalline aggregates, are zoned discontinuously reflecting variations in Mn and Zn contents. Generally, they have Mn-rich cores and Zn-dominant rims (consisting of up to 93 mol.% of the genthelvite end-member). The late-stage Be minerals are products of low-temperature alteration of primary beryl under the influence of Ca- and Mn-rich fluids.

Author info
J. Langhof & D. Holtstam, Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden, e-mail dan.holtstam@nrm.se; L. Gustafsson, Glidflyktsgatan 2II, SE-128 33 Skarpnäck, Sweden. Present address for J. Langhof: Långbans Gruvby/Värmlands museum, Hyttbacken, SE-682 92 Filipstad, Sweden, e-mail jorgen.langhof@wermlandsmuseum.se.

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