Phenakite and bertrandite; products of post-magmatic alteration of beryl in granitic pegmatites (Tatric Superunit, Western Carpathians, Slovakia)

• Published in: Mineralogical magazine Vol. 86; no. 5; pp. 715 - 729
• Main Authors: Uher, Pavel, Ozdin, Daniel, Bacik, Peter, Stevko, Martin, Ondrejka, Martin, Rybnikova, Olena, Chladek, Stepan, Fridrichova, Jana, Prsek, Jaroslav, Puskelova, Lubica
• Format: Journal Article
• Language: English
• Published: London Cambridge University Press on behalf of the Mineralogical Society of the UK and Ireland 01.10.2022; Cambridge University Press
• Subjects: alkali feldspar; alteration; bertrandite; beryl; Beryllium; Carpathians; cathodoluminescence; Central Europe; clay minerals; Crystals; Europe; feldspar group; framework silicates; granites; hydrothermal alteration; igneous rocks; Investigations; K-feldspar; Metamorphic rocks; metasomatism; mica group; Mineralogy; Minerals; muscovite; nesosilicates; orthosilicates; pegmatite; Petrography; phenakite; phenakite group; plutonic rocks; Quartz; Raman spectra; ring silicates; sheet silicates; silica minerals; Silicates; Slovakia; Slovakian Carpathians; sorosilicates; spectra; Tatra Mountains; Tatric Superunit; Trace elements; Western Carpathians; X-ray diffraction; X-ray diffraction data; Slovakia
• ISSN: 0026-461X; 1471-8022
• DOI: 10.1180/mgm.2022.99

The beryllium silicate minerals phenakite and bertrandite have been identified in granitic pegmatite dikes of the beryl-columbite subtype of Variscan age (∼340-355 Ma), associated with S- to I-type granitic rocks of the Tatric Superunit, Western Carpathians (Slovakia). The two beryllium silicates and associated minerals were characterised by electron microprobe analysis, back-scattered electron petrography and cathodoluminescence imagery, X-ray diffraction and micro-Raman techniques. Phenakite and bertrandite form euhedral-to-anhedral crystals and aggregates in irregular domains and veinlets replacing primary magmatic beryl. A detailed textural study revealed a close genetic association of phenakite and bertrandite with secondary fine-grained quartz, K-feldspar and muscovite. Locally, clay phyllosilicate minerals, (with compositions similar to those of Fe-dominant hydrobiotite, beidellite, nontronite and saponite) occur as the youngest minerals. During the post-magmatic (hydrothermal) stage of the pegmatites, infiltration of aqueous K-bearing fluids at T ≈ 200-400°C resulted in the breakdown of magmatic beryl to secondary assemblages containing phenakite and bertrandite.