Polychromatic polarization: new spectacles for the good old petrographic microscope B. Cesare

Polychromatic polarization: new spectacles for the good old petrographic microscope Cesare, B.1 and Shribak, M.2 1Department of Geosciences, University of Padova. Via Gradenigo 6, 35131 Padova, Italy. Email: bernardo.cesare@unipd.it 2Marine Biological Laboratory, 7 MBL St, Woods Hole, MA 02543, USA ___________________________________________________________________________ The polarizing microscope, the fundamental tool for any first characterization of geological materials, suffers from one major limitation, namely the poor ability to image microstructures where minerals have a retardance less than 400 nm and display interference colors in the gray scale. This problem, so far considered intrinsic and unsolvable, has prevented detailed optical observation of many low-birefringence (e.g., quartz, feldspars, leucite) or quasi-isotropic (e.g., garnet) rock-forming minerals. For these, alternative microscopic techniques, mostly electronic, have been developed and are routinely used. Polychromatic polarization microscopy (PPM, [1]) is a new optical technique that overcomes the above limitations and allows for the inspection of materials with retardations between 1 and 400 nm. This is achieved by means of a full spectrum color palette, where the hue depends on orientation of the slow axis and the saturation depends on the retardance amount. We have applied the PPM technique to regular 30 µm rock thin sections, with a particular interest for the subtle birefringence of garnet, due both to non-cubic growth [2] or to strain induced by external stresses or by mineral inclusions. The PPM produces striking, colorful images that highlight different types of microstructures in very low retardance phases, which are virtually undetectable by conventional polarizing microscopy (Figure 1). The PPM will open new avenues for microstructural analysis of geological materials. On one hand, the direct detection and imaging of microstructures will provide a fast and cheap alternative (or complement) to time-consuming and more expensive SEM-based analyses such as, e.g., EBSD. On the other hand, this powerful imaging method will provide – again in a very fast way – a much better texturally constrained basis for the location of targets for cutting-edge applications such as, e.g., FIB-TEM or Atom Probe.