Spectral reflectance of carbonate minerals in the visible and near infrared (0.35–2.55 um): Anhydrous carbonate minerals

Abstract

Reflectance spectroscopy in the visible and near infrared (0.35–2.55 um) offers a rapid, inexpensive, nondestructive technique for determining the mineralogy and gaining information on the minor element chemistry of carbonate minerals and rocks. Spectra of all commonly occurring anhydrous end‐member carbonate minerals contain seven strong absorption bands at wavelengths >1.6 um due to vibrations of the carbonate radical. Positions, widths, and spacing between carbonate bands are diagnostic of mineralogy. Differences in positions of carbonate bands between spectra of different minerals are primarily due to differences in mass of the major cation, with cation electronegativity playing a secondary role. Spectra of calcite group minerals may contain absorption features due to transition metal cations such as Fe and Mn, which can also aid in mineral identification. Spectra confirm the occurrence of both cations in the divalent state. Fe 2+ produces a broad feature near 1.1 um whose position and degree of doubling are related to the size and degree of distortion of the octahedral site, as predicted by crystal field theory. Mn 2+ and Fe 2+ produce very strong absorption bands in the minerals for which they are the major cation (rhodochrosite and siderite, respectively), making it easy to distinguish these from other carbonate minerals, even when presence of water bands may make it difficult to determine accurately carbonate band positions.