Phenakite with Tourmaline Inclusion Containing Spiral Growth
A helix stands in high relief against the orangy pink of a well-defined tourmaline crystal—impressive screw dislocation growth is one of the main identifying characteristics detailing the inner world of a phenakite specimen from Ambohimanambola, Madagascar (see above).
Previously in the collection of longtime California mineral collector Kay Robertson, the unique inclusions in this gem were discovered during digitization for the Los Angeles Museum of Natural History’s Gem and Mineral Lab. After spotting a series of parti-color tourmaline needles with vivid interference patterns under crossed polarizers, magnification was subsequently increased to investigate further. Soon it became clear that the needles scattered throughout the phenakite crystal exhibited strongly pleochroic colors as well as distinctive growth features.
In the absence of polarized light, the needles appeared in desaturated orange to red and green to blue hues. When magnification was increased once again, the eclectic world of this phenakite unveiled another surprise: The tourmaline inclusions were not alone. Alongside and extending from the base of the striated needles were colorless euhedral crystals exhibiting hexagonal form and inclusions of their own. One pair of primary inclusions, a needle anchored to another stubby crystal, were of particular interest. Raman spectroscopy confirmed the identity of the needle-like inclusions as tourmaline, though it was not possible to confirm the identity of the colorless crystal.
The pictured set of inclusions features an orangy pink tourmaline emerging from the body of a colorless crystal. The host specimen’s high clarity highlights the growth features both inside and on the surface of its inclusions, not only showing striations parallel to the c-axis typical of tourmaline, but also more unusual “spiral” or screw dislocation growth. The entwined pair are crossed in the background by two larger dark greenish blue tourmaline needles that provide a striking geometric backdrop. Dislocation spirals like those seen here are formed when planes of atoms within a gemstone’s crystal lattice are displaced during growth and rearrange themselves in a helical pattern corresponding to the direction of stress.
Perhaps best known for their intricate liddicoatite crystals, the Anjanabonoina pegmatites in Madagascar also produce a wide array of other gem minerals such as phenakite, beryl, spodumene, and danburite, among others. This particular stone presents an excellent example of the diverse mineralization that occurs at Anjanabonoina, showing off the habit, optical properties, and growth features of two important gem silicates from this gemologically important locality.
