Geology and Gemology
Spectroscopy is a powerful tool for the investigation of the composition of rocks and gems. Naturally occurring minerals containing uranium have been studied using both Raman and photoluminescence (PL) spectroscopy, where it is possible to extract distinct spectral signatures of the numerous minerals arising from the large coordination numbers arising from the four oxidation states of uranium [1]. Raman spectroscopy has been useful in the study of gems and provides clear signatures of common gems (aquamarine, sapphire) as well as unusual cordierite gems (tanzanite, forsterite, iolites). Gems belong to a mineral group constituting an isomorphic series with changing chemical compositions, and Raman spectroscopy is ideal for studying their composition [2]. Likewise, PL spectroscopy has broad applications in gemology, and is especially relevant for diamond, where nearly all type II colorless to near-colorless and fancy-color diamonds require PL analysis for definitive characterization as natural, treated, or synthetic [3]. Most studies using PL for minerals and rocks employ UV light sources, as nearly all geological samples of interest exhibit fluorescence from UV illumination. In particular, diamond, with its wide 5.5 eV bandgap, would require a 225nm laser source for excitation. However, longer wavelength sources can capture PL emissions from defect states lying within the bandgap and providing unique signatures of their presence.
Nearly all spectroscopy studies of minerals and rocks are single-shot measurements, where the laser source is directed at a particular site of the sample and the spectrum is collected. In this way, it is possible to capture a number of PL emission peaks that are characteristic of a particular compound. Sites containing multiple compounds at a single spatial location (but perhaps at different depths) may yield spectra that are highly overlapped and difficult to deconvolve, leading to ambiguity in their identification. Likewise, lateral separations of order microns of distinct compounds may or many not be captured by single-shot PL.
To study the complex distributions of minerals in a rock, scanning methods are essential to capture PL spectra across a spatial area of order mm-cm square and from various layers comprising the rock. Once such spectra are captured, an additional step is required, namely the application of precision deconvolving software to enable extraction of otherwise unresolved or overlapping emission peaks. The Klar scanning confocal microscope system, with its modular design enabling a wide range of probe wavelengths, its precision fitting and analysis software, and its auto-focus scanning, offers the geologist or gemologist a new precision tool for investigation and characterization of minerals and rocks, their composition, and their defects and impurities.
Capability of the Klar platform
A well-known source of minerals of interest in geology is the Franklin and Sterling (FS) mineral deposits of New Jersey. Minerals from FS number in the hundreds, and nearly a third exhibit luminescence. Cathodoluminescence and electron-probe microanalysis have been employed in extensive investigations to obtain point spectra and images of samples from FS (P.K. Carpenter and E.P. Vicenzi, Cathodoluminescence 2011, AMAS, p. 17). The deposit is comprised of two zinc ore bodies formed by metamorphism of limestone and gneiss Three basic mineral suites are present, banded Zn ore containing zincite, franklinite, willemite, Mn-bearing minerals, and calcite; calc-silicate assemblages which include Mn and Pb-bearing minerals, and diverse late-stage alteration assemblages.
Franklinite itself is a black non-fluorescent mineral, often associated with brilliantly fluorescent calcite and willemite, and also occurs with red or orange zincite, providing very colorful specimens. Franklinite is sometimes magnetic, but is a distinct mineral species from magnetite. Most luminescence has been observed in hand specimens using UV sources and spectacular samples containing up to five luminescent minerals are not uncommon. Such samples are ideal for evaluating the Klar scanning spectroscopic microscope capabilities.
A sample of rock from the Franklin and Sterling (FS) mineral deposits in New Jersey was investigated using the Klar system and is shown below, encased in epoxy. This particular sample was obtained from Dr. Tom Williams of the University of Idaho.
PL scans using a 349nm excitation laser, at 3 mW on the sample surface, were performed using a 10 micron step size, generating 3,481,731 spectra, each from a sub-micron spot on the sample. Up to five PL emission peaks were present in each spectrum. Klar’s software, KlarFit, using a five peak model (3 Gaussian, 2 bi-Gaussian) to extract the wavelengths (or energies) at each peak, creating the rather large dataset of the five peak energies and the intensity at each peak over each of the 3,481,731 samples. This dataset was then used to create multiple maps of the intensity and energy of each peak over the surface of the sample. Some of the peaks are overlapping and can only be resolved using KlarFit.
The map below shows the intensity across the sample at the 525nm wavelength corresponding to the PL emission of Willemite, and the variation in wavelength of the peaks across the sample. The greenish regions show the characteristic emission from Mn2+ substitution for Zn2+ in Willemite.
The dataset captured by the Klar instrument included a red emission around 700nm, much weaker than the green Willemite emissions, and also a 380 nm emission corresponding to Zincite (ZnO). Some of the emission spectra from locations containing Zincite also contained Willemite. A three color plot of the overall emission intensity from the sample is shown below. It allows visualization of the distribution of the emitting mineral species, all at once.
If you are studying the purity of gems, the content of various minerals and their distribution in a rock sample, or are just curious to see what’s inside of your particular material, the Klar platform may be an ideal choice for your lab or field work.
[1] E. Faulques et al., Application of Raman and photoluminescence spectroscopy for identification of uranium minerals in the environment, Spectroscopy Europe Vol. 27, No. 1, 14-18 (2015).
[2] D. Bersani and P.P. Lottici, Applications of Raman spectroscopy to gemology, Anal. Bioanal. Chem. 397, 2631-2646 (2010).
[3] S. Eaton-Magana and C.M. Breeding, An Introduction to Photoluminescence spectroscopy for diamond and its applications in gemology, Gems & Gemology, pp. 2-17 (Spring 2016).