Chiavennite and zoned genthelvite-helvite as late-stage minerals of the Proterozoic LCT pegmatites at Uto, Stockholm, Sweden

The Uto pegmatite system is host to a plethora of late-formed minerals, mos t of which have not been detected in previous studies of this classical loc ality. Among them are minerals that developed in cracks and druses, includi ng a number of beryllosilicates, like the zeolite chiavennite, genthelvite- helvite, milarite and bavenite; Mn-rich minerals such as manganite, wickman ite and friedelite; and several sulphides. The parageneses and crystal chem istry of chiavennite and genthelvite-helvite are the focus of the present p aper. 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 (Ca0.93Na0.07)(Mn0.77Fe0.102+Ca0.07Fe0 .063+Mg0.01) [Be1.86B0.15][Si4.68Al0.32]O-12.86(OH)(2.14).2H(2)O. The corre sponding orthorhombic unit-cell parameters are a = 8.907(4), b = 31.227(9), and c = 4.777(2) Angstrom. The genthelvite-helvite grains, often arranged in polycrystalline aggregates, are zoned discontinuously reflecting variati ons in Mn and Zn contents. Generally, they have Mn-rich cores and Zn-domina nt rims (consisting of up to 93 mol.% of the genthelvite end-member). The l ate-stage Be minerals are products of low-temperature alteration of primary beryl under the influence of Ca- and Mn-rich fluids.