Common, non-cubic garnet in metamorphic rocks: the state of the art

Since the first report on the tetragonal structure of common (OH- and andradite-free) garnet in metamorphic rocks (Cesare et al., 2019) , many new occurrences have been discovered. Non-cubic anhydrous garnets are widespread in low-temperature metamorphic terranes worldwide, including the world-renowned blueschists from the Franciscan Complex (USA), Syros (Greece) and Aosta Valley (Italy), and the phyllites from the iconic Barrow garnet zone of Scotland. These garnets share compositional features such as grossular >20% and pyrope <7%, which are common in both metabasites and metapelites metamorphosed at T < ~500 °C. The non-isotropic nature of these garnets determines a very weak birefringence, which is generally overlooked, yet it can be easily detected by polychromatic polarization microscopy on conventional 30-µm thin sections. Similarly, this deviation from an isotropic behaviour can be observed by Raman microspectroscopy using crossed-polarized geometry. Optically, the birefringence pattern suggests the presence of twelve growth sectors (twins) arranged in a dodecahedron, displaying sector quartets with three crystallographic orientations having optical axes swapped by 90°. Preliminary single-crystal XRD and HRTEM investigations have demonstrated that these anisotropic garnets have a pseudocubic tetragonal structure with a minimal (~ 1/1000) difference between the a,b and c axes. Due to the minimal departure from the cubic cell, conventional EBSD orientation analysis could notdiscriminate the orthogonal orientation of axes in the optically different sectors. However, EBSD revealed consistent, small (<1°) structural disorientations coincident with the rhythmic Ca-Fe compositional variations, similar to what is observed in zircon growth zoning. This presentation will report the new results obtained from FPA-FTIR, HRTEM, single-crystal XRD and synchrotron micro-XAS investigations, aiming to better constrain the crystallographic structure of tetragonal garnets as well as their OH- content, thereby providing a better understanding of the origin of their departure from a cubic structure.   References Cesare, B., Nestola, F., Johnson, T. et al. Garnet, the archetypal cubic mineral, grows tetragonal. Sci Rep 9, 14672 (2019). https://doi.org/10.1038/s41598-019-51214-9