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

Key Words:

• Beryllium
• boron
• chiavennite
• genthelvite
• helvite
• latestage mineralization
• granitic pegmatite
• Utö
• Sweden