FTIR IDENTIFICATION OF SERPENTINE POLYMORPHS IN POLISHED SECTIONS

Serpentine is an important group of minerals as serpentinites represent the major host for water in subduction zone. For this reason, understanding serpentinites formation and evolution during subduction is critical to better constrain the fluid-rock interactions occurring in subduction zones. However, serpentine minerals (lizardite, chrysotile, antigorite) remain difficult to identify as they have the same chemical composition, but a different structure. This project examined polished sections of serpentinite samples collected in Guatemala and the East Coast of the US along the Appalachian mountains to determine different ways of identifying polymorphs. So far, serpentine polymorph determination is either conducted using X-ray Diffraction (XRD), or Raman Spectroscopy. The accuracy of XRD in identifying the different polymorphs of serpentine is discussed, and Raman spectroscopy is the most certain method to determine serpentine polymorphs. Because Raman spectroscopy and Fourier Transform Infrared (FTIR) spectroscopy are similar, this project aimed to use a FTIR to examine the serpentine in polished sections. This method will allow for quick determination of the minerals within serpentine-bearing rocks and allow for the elucidation of formation conditions, while preserving petrographic context.

Samples were analyzed using a Thermo i10 Mx micro-FTIR spectrometer equipped with a liquid nitrogen cooled MCT detector housed at the American Museum of Natural History. This work was conducted as part of the NSF sponsored REU program in 2020, which was entirely remote including operation of the spectrometer. We measured 283 grains from 8 polished mounts from Guatemala and the East Coast of the US.

We found that the FTIR could successfully generate polymorph specific spectra and the orientation effects (10-30 cm^-1) caused by the difference in grain position within the section was not inhibitive of the identification due to the large structural differences (75-100 cm^-1). Analysis of the spectra led to identification of multiple polymorphs not initially found by XRD scans of the samples. This suggests that FTIR analysis could be a potential fast and easy method of polymorph identification by crystallographic structure. The final step of this study will be to compare FTIR and Raman spectroscopy results.