alexandrite, beryl, emerald, phenakite, Franqueira, Spain, gems

Marcos-Pascual and Moreiras examine the structural, chemical, physical and microscopic characteristics of emerald, alexandrite and phenakite from Franqueira in northwestern Spain, documenting an unusual beryllium-mineral association and assessing its gemological potential. Mineralization occurs in phlogopite-rich metasomatic rock developed where fluid-rich granitic pegmatites interacted with dunite. Beryllium supplied by the pegmatites combined with chromium derived from the ultramafic rock to produce emerald and alexandrite. The study employs single-crystal X-ray diffraction, electron microprobe analysis, refractometry, hydrostatic weighing, infrared spectroscopy and microscopy, supplemented by determinations of lithium, beryllium and water in emerald. Emerald is the most abundant beryllium mineral, forming light to medium green prismatic crystals up to 10 cm long. Most material is translucent and unevenly coloured, although distinct growth-related colour zoning was not observed. Chemical and structural data indicate predominantly octahedral substitution in the beryl structure, with channel alkalis contributing to charge compensation. Infrared spectra show that alkali-associated Type II water predominates over Type I water. Common inclusions comprise brown iron-rich phlogopite, negative crystals containing fluid phases, and healed fractures; less frequent mineral inclusions include quartz, pyrite, phenakite, alexandrite, ilmenite and anthophyllite. Previously reported fluid studies distinguish methane-bearing, carbon-dioxide-bearing and secondary saline aqueous inclusions. Alexandrite forms twinned crystals and skeletal inclusions in emerald, displaying a pronounced change from green or bluish-green in daylight to purplish-red under incandescent illumination, together with strong pleochroism. Abundant phlogopite inclusions and fractures limit the availability of good-quality cut stones. Phenakite is less abundant but can be transparent, colourless and suitable for faceting; its inclusions also include phlogopite and fluid-bearing healed fractures. Comparison with the genetically similar Tokovaja deposits in the Ural Mountains reveals broadly comparable alexandrite and phenakite properties, but differences in emerald colour, chemistry, infrared band intensities and inclusion morphology. Franqueira emeralds are generally paler because of lower chromium contents and lack several channel-like inclusion structures described in Uralian material. The authors relate these differences partly to the much smaller exposed extent and narrower geological diversity of Franqueira, while allowing that mineralization may continue underground. The study establishes a reference for identifying and understanding material from a locality vulnerable to depletion by collecting.