Identification of Uranium Minerals in Natural U-Bearing Rocks Using Infrared Reflectance Spectroscopy

Abstract

The identification of minerals, including uranium-bearing species, is often a labor-intensive process using X-ray diffraction (XRD), fluorescence, or other solid-phase or wet chemical techniques. While handheld XRD and fluorescence instruments can aid in field applications, handheld infrared (IR) reflectance spectrometers can now also be used in industrial or field environments, with rapid, nondestructive identification possible via analysis of the solid's reflectance spectrum providing information not found in other techniques. In this paper, we report the use of laboratory methods that measure the IR hemispherical reflectance of solids using an integrating sphere and have applied it to the identification of mineral mixtures (i.e., rocks), with widely varying percentages of uranium mineral content. We then apply classical least squares (CLS) and multivariate curve resolution (MCR) methods to better discriminate the minerals (along with two pure uranium chemicals UO and UO) against many common natural and anthropogenic background materials (e.g., silica sand, asphalt, calcite, K-feldspar) with good success. Ground truth as to mineral content was attained primarily by XRD. Identification is facile and specific, both for samples that are pure or are partially composed of uranium (e.g., boltwoodite, tyuyamunite, etc.) or non-uranium minerals. The characteristic IR bands generate unique (or class-specific) bands, typically arising from similar chemical moieties or functional groups in the minerals: uranyls, phosphates, silicates, etc. In some cases, the chemical groups that provide spectral discrimination in the longwave IR reflectance by generating upward-going (reststrahlen) bands can provide discrimination in the midwave and shortwave IR via downward-going absorption features, i.e., weaker overtone or combination bands arising from the same chemical moieties.

Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)2SO4 from Single-Angle Reflectance

“…Both the real, n (ṽ∼), and imaginary, k (ṽ∼), components of the complex index of refraction, n^ = n + i k , are vital input parameters for modeling the reflectance, refraction, absorption, and emissivity of a material 1–6 and thus serve as valuable reference data. A common method of determining n and k of solid materials is to measure the change in amplitude of light reflected from a material as a function of frequency, R(ṽ∼), over a broad wavelength range, and then apply the Kramers–Kronig transform (KKT) to derive the optical constants.…”

Hyperspectral imaging of minerals in the longwave infrared: the use of laboratory directional-hemispherical reference measurements for field exploration data

“…In fact, the ðNH 4 Þ 2 HPO 4 spectrum displays some distinct features that can be used for identification, namely the NH 4 þ cation peak at 970 cm −1 , as well as the 1060, 1100 cm −1 doublet that we have previously associated as being two of the phosphate anion band modes. 27 As to the strong reflectance peaks associated with symmetric cations 46,[63][64][65] such as NH 4 þ or UO 2 2þ as well as anions 27,44 such as PO 4 3− or SO 4 2− , these peaks arise from the first surface scattering of reststrahlen bands. As we have previously reported in studies that quantified reflection versus wavelength as a function of particle size, 44 the amplitude of the other bands relative to the reststrahlen bands varies significantly with particle size.…”

“…Spectra for the three mixtures, ammonium phosphate dibasic, sodium phosphate dibasic, and the Nalgene ® lid, were recorded using a Bruker Optics IR Cube FTIR with the sphere bolted on the side as previously described. 27 A Bruker Tensor 37/A562 integrating sphere combination was used to record data for the remaining 15 samples. The spectrometer provided the modulated IR beam as input; the sphere was a Bruker A562 device, a two-port 75-mm matte-gold-coated sphere with a dedicated detector.…”

“…Many HSI studies followed in the 1980s and 1990s 21 including, e.g., the first TIR imaging studies by Kahle et al 22 as well as several others. In many of these pioneering works, it was noted that minerals containing anion chemical moieties such as silicates, 23 carbonates, 24 phosphates, 25 sulfates, 26 or cations such as ammonium or uranyl 27,28 are all known to display characteristically strong emission/reflectance features in the LWIR between 7 and 13 μm, making them amenable to such TIR detection.…”

Practical model for improved classification of trace chemical residues on surfaces in active spectroscopic measurements

“…One of the more widely accepted approaches is to use single-angle reflectance spectroscopy followed by Kramers-Kronig transformation for estimating optical constants (ñ) for crystalline solids. [21][22][23][24] For solid minerals, DeVetter et al have shown that using carefully designed mask apertures with low reflectance is more optimal for solid minerals. 25 The optical constants used in this research were measured and provided by Pacific Northwest National Laboratory (PNNL) through IARPA's (Intelligence Advanced Research Projects Activity) SILMARILS (Standoff ILluminator for Measuring Absorbance and Reflectance Infrared Light Signatures) program.…”