The beryllium silicate minerals phenakite and bertrandite have been identified in granitic pegmatite dykes 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.

The onset of melting of beryl + H2O was determined experimentally in the P-T range 10–25 kbar, 900 to 1200°C, by synthesis runs on the composition Al2Be3Si6O18. Beryl begins to melt incongruently to chrysoberyl, phenakite, and melt at temperatures slightly above 1000°C. At higher temperatures a divariant stability field exists where beryl, chrysoberyl, phenakite, and melt coexist due to the solubility of H2O in beryl and in the melt. The final disappearance of beryl was bracketed between 1050 and 1100°C at 10 kbar and at 1100°C between 10 and 20 kbar. The phases (analysed for Al and Si by electron microprobe) do not show major deviation from stoichiometric composition.