Raman Microspectroscopic Mapping with Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) Applied to the High-Pressure Polymorph of Titanium Dioxide, TiO-II.

The high-pressure, α-PbO-structured polymorph of titanium dioxide (TiO-II) was recently identified in micrometer-sized grains recovered from four Neoarchean spherule layers deposited between ∼2.65 and ∼2.54 billion years ago. Several lines of evidence support the interpretation that these layers represent distal impact ejecta layers. The presence of shock-induced TiO-II provides physical evidence to further support an impact origin for these spherule layers. Detailed characterization of the distribution of TiO-II in these grains may be useful for correlating the layers, estimating the paleodistances of the layers from their source craters, and providing insight into the formation of the TiO-II. Here we report the investigation of TiO-II-bearing grains from these four spherule layers using multivariate curve resolution-alternating least squares (MCR-ALS) applied to Raman microspectroscopic mapping. Raman spectra provide evidence of grains consisting primarily of rutile (TiO) and TiO-II, as shown by Raman bands at 174 cm (TiO-II), 426 cm (TiO-II), 443 cm (rutile), and 610 cm (rutile). Principal component analysis (PCA) yielded a predominantly three-phase system comprised of rutile, TiO-II, and substrate-adhesive epoxy. Scanning electron microscopy (SEM) suggests heterogeneous grains containing polydispersed micrometer- and submicrometer-sized particles. Multivariate curve resolution-alternating least squares applied to the Raman microspectroscopic mapping yielded up to five distinct chemical components: three phases of TiO (rutile, TiO-II, and anatase), quartz (SiO), and substrate-adhesive epoxy. Spectral profiles and spatially resolved chemical maps of the pure chemical components were generated using MCR-ALS applied to the Raman microspectroscopic maps. The spatial resolution of the Raman microspectroscopic maps was enhanced in comparable, cost-effective analysis times by limiting spectral resolution and optimizing spectral acquisition parameters. Using the resolved spectra of TiO-II generated from MCR-ALS analysis, a Raman spectrum for pure TiO-II was estimated to further facilitate its identification.