Tanzania, scapolite, gems

Uses heating experiments and optical observations to refine the interpretation of coloured inclusions and post-crystallisation conditions in scapolite from central Tanzania. The study follows earlier investigations of gem-quality Tanzanian scapolites noted for their homogeneity, high birefringence and occasional chatoyancy. Parallel needle-like channels had previously been associated with reddish iron oxide or hydroxide material and considered responsible for the cat’s-eye effect. Electron-microprobe work subsequently showed that material occupying the growth tubes could have the same composition as the surrounding scapolite, while the yellow and brownish-red laminae within them represented separate mineral species. Yellow laminae were tentatively assigned to lepidocrocite from their appearance, colour and crystal habit. The reddish scales could be either maghemite or hematite, potentially produced through dehydration of lepidocrocite under different temperature conditions. To constrain this distinction, the authors draw on experimental observations of transparent scapolites from several geological settings. Refractive indices were measured during heating to 450 °C and again at room temperature after heating to temperatures as high as 800 °C, below the stated decomposition limit of scapolite. Both principal refractive indices increased approximately linearly up to about 350–375 °C, with a slightly greater increase in the extraordinary index. The magnitude of the change was approximately related to meionite content. At higher temperatures, a sharper increase in both indices was attributed to water loss. Measurements after cooling yielded nearly constant room-temperature values across the investigated heating range. Colour behaved differently: its intensity progressively diminished, and specimens became colourless at approximately 350–400 °C. The authors apply this experimentally observed colour loss to a faintly yellow Tanzanian specimen containing reddish laminae. Because its colour remained intact, they infer that it had not experienced comparable high temperatures after formation. This inferred thermal limit favours maghemite over hematite as the identity of the reddish material in its growth channels. The paper therefore combines analytical observations, experimental colour changes and proposed iron-mineral transformations to narrow an otherwise uncertain inclusion identification and reconstruct part of the specimen’s thermal history.