Garnets, archetypal cubic minerals, do not grow cubic
Garnet is the archetypal cubic mineral, stable to temperatures approaching 2000°C and pressures of ~25 GPa, and occurring in a wide variety of rock types in Earth’s crust and upper mantle. Owing to its prevalence, durability and compositional diversity, it is used to investigate a broad range of geological processes. Common garnet is anhydrous and isotropic; the optical anisotropy (Fig. 1) occasionally observed in some crystals is attributed to internal strain of the cubic structure. A set of samples from different geological occurrences by using a multi-technique approach including optical microstructural analysis, BSEM, EMPA, EBSD, FPA-FTIR, TEM and single-crystal XRD have been studied.
Fig. 1 One type of birifrangent garnets studied.
Investigations using HRTEM were performed for all samples with ordered and undeformed crystal structure, having very limited strain or defects. As birefringence in garnet may be related to the presence of OH or H2O in the mineral, the sector-zoned garnets from Cazadero, the eastern Alps and Pfitscher Jochwere analysed by FTIR imaging (Fig. 2), collecting both single-spot maps and FPA (focal-plane-array detector) images at SINBAD, INFN-LNF (Frascati).
The results show that, although garnet in the Cazadero sample contains 100–300 μm-long needles and lamellae of hydroxylated minerals, the garnet itself is anhydrous within the limits of the technique (few ppm). This study shows that common garnets with a non-cubic (tetragonal) crystal structure are much more widespread than previously considered, occurring in low-temperature, high-pressure metamorphosed basalts (blueschists) from subduction zones and in low-grade metamorphosed mudstones (phyllites and schists) from orogenic belts. The model proposed was that in most cases garnets do not initially grow cubic, and this conclusions implies that the crystal chemistry and thermodynamic properties of garnets at low-temperatures need to be re-assessed, with potential consequences for its application as an investigative tool in a broad range of geological environments.
Fig. 2 High resolution FTIR image consisting of a mosaic of 15 single FPA images taken at the SINBAD beamline.
 Cesare B., Nestola F., Johnson T., Mugnaioli E., Della Ventura G., Peruzzo L., Bartoli O., Viti C., Erickson T. Scientific Reports (2019) 9:14672, DOI: 10.1038/s41598-019-51214-9
Prof. Giancarlo Della Ventura
Dipartimento di Scienza, Università Roma Tre