Analysis of Minerals Using Linearly Polarized Infrared Microspectroscopy

This is an extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Collecting Quality Infrared Spectra from Microscopic Samples of Suspicious Powders in a Sealed Cell

The infrared (IR) microspectroscopical analysis of samples within a sealed-cell containing barium fluoride is a critical need when identifying toxic agents or suspicious powders of unidentified composition. The dispersive nature of barium fluoride is well understood and experimental conditions can be easily adjusted during reflection–absorption measurements to account for differences in focus between the visible and IR regions of the spectrum. In most instances, the ability to collect a viable spectrum is possible when using the sealed cell regardless of whether visible or IR focus is optimized. However, when IR focus is optimized, it is possible to collect useful data from even smaller samples. This is important when a minimal sample is available for analysis or the desire to minimize risk of sample exposure is important. While the use of barium fluoride introduces dispersion effects that are unavoidable, it is possible to adjust instrument settings when collecting IR spectra in the reflection–absorption mode to compensate for dispersion and minimize impact on the quality of the sample spectrum

Infrared Absorption Investigations Confirm the Extraterrestrial Origin of Carbonado-Diamonds

The first complete infrared FTIR absorption spectra for carbonado-diamond confirm the interstellar origin for the most enigmatic diamonds known as carbonado. All previous attempts failed to measure the absorption of carbonado-diamond in the most important IR-range of 1000-1300 cm-1 (10.00-7.69 micro-m.) because of silica inclusions. In our investigation, KBr pellets were made from crushed silica-free carbonado-diamond and thin sections were also prepared. The 100 to 1000 times brighter synchrotron infrared radiation permits a greater spatial resolution. Inclusions and pore spaces were avoided and/or sources of chemical contamination were removed. The FTIR spectra of carbonado-diamond mostly depict the presence of single nitrogen impurities, and hydrogen. The lack of identifiable nitrogen aggregates in the infrared spectra, the presence of features related to hydrocarbon stretch bonds, and the resemblance of the spectra to CVD and presolar diamonds indicate that carbonado-diamonds formed in a hydrogen-rich interstellar environment. This is consistent with carbonado-diamond being sintered and porous, with extremely reduced metals, metal alloys, carbides and nitrides, light carbon isotopes, surfaces with glassy melt-like patinas, deformation lamellae, and a complete absence of primary, terrestrial mineral inclusions. The 2.6-3.8 billion year old fragmented body was of asteroidal proportions

FT-IR microscopical analysis with synchrotron radiation: The microscope optics and system performance

When a Fourier transform infrared (FT-IR) microspectrometer was first interfaced with the National Synchrotron Light Source (NSLS) in September 1993, there was an instant realization that the performance at the diffraction limit had increased 40-100 times. The synchrotron source transformed the IR microspectrometer into a true IR microprobe, providing high-quality IR spectra for probe diameters at the diffraction limit. The combination of IR microspectroscopy and synchrotron radiation provides a powerful new tool for molecular spectroscopy. The ability to perform IR microspectroscopy with synchrotron radiation is still under development at Brookhaven National Laboratory, but several initial studies have been completed that demonstrate the broad-ranging applications of this technology and its potential for materials characterization

Rapid, Reliable and Reviewable Mineral Identification with Infrared Microprobe Analysis

Infrared microprobe analysis (IRMA) is a valuable technology for mineral identification. Prior to the discovery of X-ray diffraction, the polarised light microscope was the principal tool for identifying minerals. While X-ray diffraction and electron beam microanalysis made major contributions to mineralogy, IRMA contributes unique information about the chemistry of complex minerals. Using diamond internal reflection to collect infrared (IR) spectra of minerals is an approach that extends optical mineralogy to new levels. It is simple, direct and rapid. Traditional whole grain mounts or petrographic sections, used for polarized light microscopy, are analysed by IR internal reflection. Attenuated total reflection (ATR) spectra are collected by touching the diamond internal reflection element to areas of interest. Silicates, carbonates, nitrates, phosphates, hydrates and other minerals provide unique spectra for identification. Using the diamond ATR microscope objective allows for the selective isolation of individual minerals for simultaneous collection of microscopic, optical and IR data, thus enabling the indisputable identification of most minerals. Applying IRMA to forensic soil analysis is an example of using this IR spectral method. A library of diamond ATR spectra of common soil minerals was prepared. These reference data let the analyst confirm the identity of mineral grains in soil evidence. Polarised light microscopical examination is the primary step. Each phase is then characterized by its ATR spectrum and searched through the spectral library. IRMA unites light microscopy with IR spectroscopy to create a powerful system capable of identifying mineral grains and correlating microstructure with chemistry. This technique has great utility in the analysis of material in a forensic context, particularly in relation to trace evidence