- © 2014 Mineralogical Society of America
Optical spectroscopy is concerned with the measurement of the absorption, reflection and emission of light in the near-ultraviolet (~250 nm) through the mid-infrared (~3000 nm) portions of the spectrum. The human interface to the geological and mineralogical world is primarily visual. Optical spectroscopy is, in particular, well suited to investigating the origin of color in minerals. The reflection spectroscopy of minerals has been motivated to a large extent by interest in remote sensing. Emission spectra are usually studied in reference to luminescence phenomena. Studies of mineral color, metal ion site occupancy, oxidation states and concentrations have generally been done with absorption spectroscopy. This chapter concentrates on single crystal absorption spectroscopy.
Absorption of light by crystals can occur for a number of reasons. For many minerals, the presence of ions of transition elements (e.g., Ti, V, Cr, Mn, Fe, Co, Ni, Cu) in their various oxidation states is the cause of light absorption. In some minerals, the individual ions cause the light absorption while in others it is the interaction between ions such as between Fe2+ and Fe3+ that causes color. In some minerals, rare-earth elements are an important source of color. Some minerals are colored by small molecular units involving metal ions (UO22+, CrO42−) or anions (S3− in sodalites). Many sulfide minerals such as cinnabar (HgS) and realgar (As4S4) owe their color to band gaps in the semiconducting sulfides. Other important sources of color in minerals are the products of radiation damage which can be metal ions that have changed oxidation states, electron vacancies (called “hole” centers), or unpaired electrons located on crystal defects or on ions that are not normally associated with unpaired electrons. All of these are commonly studied, in part, with optical spectroscopy.
Historically, the …