EGU21-14155
https://doi.org/10.5194/egusphere-egu21-14155
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Even the low-T garnet from the iconic Barrow’s zone is tetragonal

Bernardo Cesare and Fabrizio Nestola
Bernardo Cesare and Fabrizio Nestola
  • Department of Geosciences, University of Padova, Padova, Italy (bernardo.cesare@unipd.it)

Common (anhydrous) Fe-Mg-Ca-Mn garnet, the archetypal cubic mineral, has been recently discovered to be tetragonal in metapelites and metabasites from low-temperature regional metamorphic terranes (Cesare et al., 2018).

Despite the differences in bulk rock composition and pressure conditions, such low-T tetragonal garnets share common chemical features, namely high grossular (>25 mol%) and low pyrope (<7 mol%) contents. Similar compositions are documented in other contexts worldwide, both in blueschists-eclogites and in phyllites, including the metapelites from the garnet zone of the iconic Barrovian metamorphism of the Scottish highlands (Viete et al., 2011).

We have analysed a garnet crystal from a chlorite-biotite schist collected at the Barrow’s garnet zone in Glen Esk. The unit cell parameters were refined using diffraction reflections between 1.20 and 0.55 Å providing a tetragonal cell with a = 11.5731(5) Å and c = 11.5887(8) Å and volume V = 1552.15(15) Å3. Systematic absences analysis on complete intensity data collected up to 2theta = 80° indicated I41/acd space group confirming the cell parameters refinement.

Therefore, the garnet is tetragonal and not cubic, as suggested by its weak birefringence under crossed polarizers.

These results show that the tetragonal structure of common Fe-Mg-Ca-Mn garnet is verified whenever this mineral displays the Ca-rich, Mg-poor composition often observed in low-T metamorphic rocks. And support the hypothesis that the lowering of symmetry is composition-dependent.

 

References

Cesare, B., et al. Garnet, the archetypal cubic mineral, grows tetragonal. Sci Rep 9, 14672 (2019).

Viete, D.R., et al. The nature and origin of the Barrovian metamorphism, Scotland: Diffusion length scales in garnet and inferred thermal time scales. J. Geol. Soc. London 168, 115–132 (2011).

 

How to cite: Cesare, B. and Nestola, F.: Even the low-T garnet from the iconic Barrow’s zone is tetragonal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14155, https://doi.org/10.5194/egusphere-egu21-14155, 2021.

Corresponding presentation materials formerly uploaded have been withdrawn.