Diffuse reflectance mid-IR spectroscopy of rocks and minerals

Agilent provides a new capability to scientists involved in the geosciences, with the development of field-ready Fourier transform infrared (FTIR) analyzers that have been optimized for the measurement of rocks, minerals and soil. These analyzers have an integrated diffuse reflectance sampling interface to enable molecular spectra to be obtained with little or no sample preparation required; that is, they can be used directly on the specimen in the field.
FTIR is a natural complement to the x-ray fluorescence (XRF) field analyzers that have been used for years by geoscientists. Whereas the latter analyzers provide information about what elements are present, FTIR provides information about how the elements are bonded to form the mineral’s chemical composition. Thus, by using both XRF and FTIR, the geoscientist has a far more complete understanding of the specimen orsite.

Diffuse reflectance mid-IR spectroscopy of rocks and minerals

FTIR spectroscopy provides information on covalently bonded molecules and therefore a variety of rocks and minerals can be analyzed using this technique. For example carbonates, nitrates, sulfates, oxides, hydroxides, silicates, aluminosilicates, phosphates, borates, vanadates, tungstates, arsenates, molybdates and uranates all have covalently bonded atoms and thus are amenable to infrared (IR) analysis.

Rocks and minerals often contain trace elements or impurities that are unique to a specimen or locale, so the ability of the user to build their own on-board library is a valuable asset to these field FTIR analyzers. The spectra of a variety of mineral classes are shown here, which include talc (magnesium silicate), vanadinite (lead chloride vanadate), gypsum (calcium sulfate dihydrate), phosphate rock (calcium phosphate) and calcite (calcium carbonate).

The spectra are all unique and indicate the characteristic ‘fingerprint’ functional groups that are evident in the spectra, such as gypsum with the SO4 asymmetric stretch at 1140 cm-1 and its first overtone at 2240 cm-1 (broad).

Though diffuse reflectance IR spectra look different to classical transmittance spectra, the spectra are reproducible and the mineral can be readily identified from the Agilent diffuse reflectance FTIR library of rocks and minerals.

Excerpt from Agilent Technologies Application Note 5990-7797EN