Direct mineralogical imaging of economic ore and rock samples with multi-modal nonlinear optical microscopy

Mung-Chung Kao,Fu-Jen Kao,Albert Stolow,Wen-Chuan Kuo,Ankur Gogoi,Patrick H J Mercier,Andrew Ridsdale,David M Kingston,Adrian F Pegoraro

doi:10.1038/s41598-018-34779-9

Abstract

Multi-modal nonlinear optical (NLO) microscopy, including stimulated Raman scattering (SRS) and second harmonic generation (SHG), was used to directly image mineralogical features of economic ore and rock samples. In SRS/SHG imaging, ore samples generally require minimal preparation and may be rapidly imaged, even in their wet state. 3D structural details, at submicron resolution, are revealed tens of microns deep within samples. Standard mineral imaging based on scanning electron microscopy (SEM), with elemental analysis via energy dispersive X-Ray spectroscopy, was used to independently validate the mineral composition of the samples. Spatially-resolved SRS from dominant Raman-resonant bands precisely maps the locations of specific minerals contained within the samples. SHG imaging reveals locally non-centrosymmetric structures, such as quartz grains. Competing absorption and nonlinear scattering processes, however, can reduce contrast in SRS imaging. Importantly, the correlation between standard electron microscopy and multi-modal NLO optical microscopy shows that the latter offers rapid image contrast based on the mineral content of the sample.

Highlights

The need for high quality, real-time information in mineral resource characterization is growing due to both declining ore grades and demands for improved energy efficiency
Judicious use of these techniques in conjunction with complementary techniques, e.g., optical microscopy[4], cathodoluminescence (CL) microscopy and spectroscopy[8], X-Ray fluorescence[9], infrared thermography[10], etc., may overcome these limitations and significantly enhance the quality and reliability of the results. This is illustrated by digital optical microscopy (DOM) which can distinguish ores consisting of minerals of similar elemental composition, e.g. hematite and magnetite[11]
The signal generated from second harmonic generation (SHG) arises from a single beam and is detected by using a second dichroic mirror located in the backward direction, selectively directing the visible SHG emission from the sample toward a photomultiplier tube (PMT)

Summary

Introduction

The need for high quality, real-time information in mineral resource characterization is growing due to both declining ore grades and demands for improved energy efficiency. We apply a correlative microscopy methodology which combines images obtained from both SEM and multimodal NLO microscopy to map mineral features of various economically-relevant ore and other types of rock samples. Both thin sections and ground particulate samples were investigated. Important in process mineralogy applications, samples for SRS/SHG imaging (unlike SEM-EDS methods) generally require minimal preparation and may be rapidly imaged even in their wet state. This correlative microscopy approach provides more information about the sample than can be achieved by any single technique alone and, importantly, validates the SRS/SHG imaging approach.

Methods

Results

Conclusion