Polychromatic polarization: Boosting the capabilities of the good old petrographic microscope

Abstract

Polychromatic polarizing microscopy (PPM) is a new optical technique that allows for the inspection of materials with low birefringence, which produces retardance between 1 nm and 300 nm. In this region, where minerals display interference colors in the near-black to gray scale and where observations by conventional microscopy are limited or hampered, PPM produces a full spectrum color palette in which the hue depends on orientation of the slow axis. We applied PPM to ordinary 30 μm rock thin sections, with particular interest in the subtle birefringence of garnet due both to non-isotropic growth or to strain induced by external stresses or inclusions. The PPM produces striking, colorful images that highlight various types of microstructures that are virtually undetectable by conventional polarizing microscopy. PPM opens new avenues for microstructural analysis of geological materials. The direct detection and imaging of microstructures will provide a fast, non-destructive, and inexpensive alternative (or complement) to time-consuming and more costly scanning electron microscope–based analyses such as electron backscatter diffraction. This powerful imaging method provides a quick and better texturally constrained basis for locating targets for cutting-edge applications such as focused ion beam-transmission electron microscopy or atom probe tomography.

Schorl-1A from Langesundsfjord (Norway)

“…The γ-refraction index is also perpendicular to the growth direction of the prism face. A similar orientation of slow-length axis of the indicatrix has been described very recently sector-zoned garnet crystals from Port Macquarie (Australia) by Cesare et al (2022), showing anomalous birefringence by using polychromatic polarizing microscopy (Shribak 2015(Shribak , 2017. This technique allows elucidation of a very subtle variation of the retardation at low birefringence.…”

“…In Fig. 2d of Cesare et al (2022), it is clearly shown that the slow length is along the growing direction. Although this is interpreted by Cesare et al (2019) as due to a desymmetrization due to cubic to tetragonal change, it might well be related to the kinetic ordering of atoms during the growing of the garnet crystals (probably due to ordering of Fe 2+ at the Y site in the garnet structure) being perhaps related to cyclic chemical zonation along the growing front.…”

Using the traditional microscope for mineral grain orientation determination: A prototype image analysis pipeline for optic‐axis mapping (POAM)

“…For this purpose, we assume the anisotropy of the garnets to be caused by a tetragonal system. 20,74 Accordingly, the c-axes are oriented 90 • /023 • and 80 • /055 • , which is apparently parallel to the foliation azimuth. This could be explained by domainal growth through epitaxy or topotaxy following the muscovite substrate crystallographic template.…”

“…18 For example, polychromatic polarised microscopy enables a new use for imaging strained isotropic materials, lowbirefringence minerals, diatom ultrathin sections, 19 and tetragonal garnets in thin sections. 20 Optical whole-slide imaging (WSI) in automated and virtual reality microscopy generates massive datasets fast because image tiles are detected, focussed, and montaged nearly simultaneously. 21 Unlike SEM-based techniques, optical WSI does not need to accumulate and digitise a signal at each pixel as in.…”

“…There has, however, been significant progress with the optical systems, hardware and software of precision petrographic microscopes equipped with digital cameras 18 . For example, polychromatic polarised microscopy enables a new use for imaging strained isotropic materials, low‐birefringence minerals, diatom ultrathin sections, 19 and tetragonal garnets in thin sections 20 . Optical whole‐slide imaging (WSI) in automated and virtual reality microscopy generates massive datasets fast because image tiles are detected, focussed, and montaged nearly simultaneously 21 .…”