Spectral reflectance (0.35–2.5 µm) properties of garnets: Implications for remote sensing detection and characterization

The utility of spectral reflectance for identification of the main end-member garnets: almandine (Fe²⁺ ₃Al₂Si₃O₁₂), andradite (Ca₃Fe³⁺ ₂Si₃O₁₂), grossular (Ca₃Al₂Si₃O₁₂), pyrope (Mg₃Al₂Si₃O₁₂), spessartine (Mn²⁺ ₃Al₂Si₃O₁₂), and uvarovite (Ca₃Cr³⁺ ₂Si₃O₁₂) was studied using a suite of 60 garnet samples. Compositional and structural data for the samples, along with previous studies, were used to elucidate the mechanisms that control their spectral reflectance properties. Various cation substitutions result in different spectral properties that can be determine the presence of various optically-active cations and help differentiate between garnet types. It was found that different wavelength regions are sensitive to different compositional and structural properties of garnets. Crystal-field absorptions involving Fe²⁺ and/or Fe³⁺ are responsible for the majority of spectral features in the garnet minerals examined here. There can also be spectral features associated with other cations and mechanisms, such as Fe^{2 +}–Fe³⁺ and Fe^{2 +}–Ti⁴⁺ intervalence charge transfers. The visible wavelength region is useful for identifying the presence of various cations, in particular, Fe (and its oxidation state), Ti⁴⁺, Mn²⁺, and Cr³⁺. In the case of andradite, spessartine and uvarovite, the visible region absorption bands are characteristic of these garnets in the sense that they are associated with the major cation that distinguishes each: ^[6]Fe³⁺ for andradite, ^[8]Mn²⁺ for spessartine, and ^[6]Cr³⁺ for uvarovite. For grossular, the presence of small amounts of Fe³⁺ leads to absorption bands near 0.370 and 0.435 µm. These bands are also seen in pyrope–almandine spectra, which also commonly have additional absorption bands, due to the presence of Fe²⁺. The common presence of Fe²⁺ in the dodecahedral site of natural garnets gives rise to three Fe²⁺ spin-allowed absorption bands in the 1.3, 1.7, and 2.3 µm regions, providing a strong spectral fingerprint for all Fe²⁺-bearing garnets studied here. Garnets containing Mn²⁺ have additional visible (∼0.41 µm) spectral features due to ^[8]Mn²⁺. Garnets containing Cr³⁺, exhibits two strong absorption bands near ∼0.7 µm due to spin-forbidden ^[6]Cr³⁺ transitions, as well as ^[6]Cr³⁺ spin-allowed features near 0.4–0.41 µm and 0.56–0.62 µm, and ^[6]Cr³⁺ spin-allowed transitions between 0.41 and 0.68 µm. Common silicate garnet spectra, in summary, are distinct from many other rock-forming silicates and can be spectrally distinct from one garnet species to another. Iron dominates the spectral properties of garnets, and the crystallographic site and oxidation state of the iron both affect garnet reflectance spectra.