tourmaline, cat’s eye, gems

The causes of chatoyancy in tourmaline are investigated through examination of twenty-two specimens representing different colours, localities and types of elongated inclusion. Polished slabs and cat’s-eye cabochons from Brazil, the United States and Mozambique were studied by optical microscopy, X-ray powder diffraction, electron-microprobe analysis and heating-stage observation of fluid inclusions. The host crystals were assigned principally to elbaite, with fewer dravitic specimens, using unit-cell parameters because conventional refractive-index and density measurements proved insufficiently reliable in the inclusion-rich material. No close relationship was established between tourmaline colour and this structural classification. Three main inclusion types are distinguished: irregular thread-like trichites, growth tubes and solid needles. Trichites commonly contain two-phase fluids and occupy healed fractures, but their variable orientation prevents them from generating a coherent cat’s-eye effect. Chatoyancy instead arises from densely distributed tubes or needles aligned parallel to the host’s principal growth direction. Growth tubes are interpreted as primary cavities associated with growth irregularities, strain or rapid crystallisation, whereas needles are elongated crystalline inclusions incorporated during growth. Within the examined suite, tubes occur more frequently in green tourmalines, while needles predominate in pink-red and blue specimens; trichites show no comparable colour association. Microchemical analyses indicate that the most common solid material filling the tubes is tourmaline with a composition and minor-element pattern similar to those of its host. Less common fillings include prosopite and cookeite, together with iron oxides and sulphides. Opaque channel material is identified as pyrrhotite, while red-to-brown coatings and iron-rich yellow halos are attributed more tentatively to iron oxides or hydroxides. Most fine crystalline needles likewise have compositions matching tourmaline. Other straight rods are identified as epidote close to clinozoisite in composition. Columnar aggregates of subhexagonal muscovite plates are also described. Heating-stage behaviour of two-phase inclusions suggests water and carbon dioxide as principal constituents, supporting a hydrothermal setting, while several associated minerals are consistent with late pegmatitic or hydrothermal processes. The study concludes that tourmaline chatoyancy cannot be assigned to a single foreign mineral inclusion: both aligned growth channels with varied fillings and crystalline needles, commonly themselves tourmaline, can produce the effect.