beryl, inclusions, gems

Mineral inclusions in colourless, greenish and bluish beryls from Brazil, Afghanistan, Kenya and Switzerland reveal recurring assemblages that illuminate the pegmatitic environments in which their hosts formed. Microscopic observations, electron-microprobe analyses of inclusions exposed at polished surfaces, and selected X-ray examinations establish several combinations of mica, manganese-bearing minerals and niobium–tantalum oxides. A Brazilian goshenite contains a large brown phlogopite crystal associated with silvery muscovite flakes and metallic black columbite, termed niobite in the paper. A second Brazilian specimen contains a comparable association together with a dark green, barrel-shaped manganese-bearing apatite crystal. In pale blue aquamarine from Nuristan, Afghanistan, brownish isometric inclusions were identified repeatedly by microprobe analysis as spessartine. An accompanying large colourless crystal, initially resembling apatite because of its apparent hexagonal form and parallel cleavage planes, was identified optically as pseudohexagonal muscovite. The most diverse assemblage occurs in greenish-blue aquamarine from Kenya and includes conspicuous albite clusters, orange spessartine, phlogopite, and columbite–tantalite minerals. Microprobe results distinguish niobium-rich tabular inclusions from tantalum-rich acicular crystals. Small green inclusions were exposed by cutting down the host’s basal surface, and extracted fragments examined with a Gandolfi X-ray camera were identified as dravite tourmaline intimately associated with gahnite. The author presents this tourmaline–gahnite association as previously unrecorded within beryl. A Swiss aquamarine contains an orange crystal with characteristic garnet morphology, interpreted as spessartine; this identification was accepted without destructive exposure or chemical analysis to preserve the specimen. An additional aquamarine, not illustrated, contains lithium-rich mica associated with columbite. The repeated occurrence of related inclusions in beryls from widely separated localities indicates similarities in their mineral-forming environments. Columbite is considered especially informative because of its association with manganese-rich granitic pegmatites containing abundant albite and lithium minerals. Such conditions also account for accompanying spessartine and manganese-bearing apatite. Large pseudohexagonal mica crystals with prominent basal cleavage provide a particularly recognizable internal feature. The study demonstrates the value of combining inclusion morphology and optical behaviour with chemical and crystallographic evidence, both to distinguish visually similar guest minerals and to interpret the growth environment of their beryl hosts.