Iolite, Peridot, Garnet absorption spectra
Following my investigations of the visible and infrared transmission (absorbance) and visible fluorescence spectra of iolite (cordierite), I became intrigued by the presence of the strong, broad absorption band centred at about 1µm and stretching from about 800-1400nm. Since essentially no light was detected in the region of overlap of the JAZ (Vis) and NIRQUEST (IR) spectrometers, it was difficult to match the transmission scales in these two wavelength regions. What is the origin of this absorption and does it appear in other minerals?
A search through my own mineral visible spectra showed that peridot (olivine) also has essentially zero transmission at 1µm. Also, the spectrum of garnet (pyrope/alamandine) is decreasing rapidly towards the IR.
The compositions of these silicate minerals are:
Iolite: (Mg_2 Al_4 Si_5 O_18) with some Mg replaced by ferrous iron (Fe^2+) and manganese, and Al by ferric iron (Fe^3+)
Peridot: (Mg, Fe)_2 SiO_4
Garnet: 3MgO/3FeO Al_2O_3 3SiO_2, Mg -> pyrope; Fe -> alamandine
I then measured the infrared transmissions of peridot (a tumble-polished fragment) and garnet (a light purple/red rough pebble) to produce these transmission and absorbance spectra. Due to the nature of the specimens, the transmission measurement have a somewhat arbitrary scale due to refraction and scattering effects and so should be considered only representative of true absorbance measurements. The relative transmission scale of the peridot in the Vis and IR is not well-determined but that of the garnet is better.
The two plots here show the result of these and the previous iolite measurements for a sample ~.8mm deep. They show the transmission vs. wavelength (in nm) and the absorbance (=-Log_10(transmission) called optical density) vs. wavenumber (with the scale reversed to keep blue on the left). The peridot has a similar - but stronger - 1µm absorption (actually centred closer to 1.1µm) stretching from ~ 700-1600nm but with very low absorption at longer wavelengths. The garnet has the characteristic complex iron spectrum in the visible but very low transmission beyond 1100nm
The common feature of these three minerals is that they are all idiochromatic (self-coloured) due to iron as part of their basic structure. Most of the absorption features are due to ferric or ferrous iron. The peridot has the 1µm feature attributed to a ligand field transition in Fe2+ (Farrell and Newnham, 1965, The American Mineralogist, 50 1972; www.minsocam.org/ammin/AM50/AM50_1972.pdf ).
The high transmission of peridot (olivine) in the IR is thought to have a significant effect on heat transfer in the Earth's mantle by providing a channel for radiative transfer.
Note that the term 'pebble' in the legend of the lower plot refers to the garnet.
Iolite, Peridot, Garnet absorption spectra
Following my investigations of the visible and infrared transmission (absorbance) and visible fluorescence spectra of iolite (cordierite), I became intrigued by the presence of the strong, broad absorption band centred at about 1µm and stretching from about 800-1400nm. Since essentially no light was detected in the region of overlap of the JAZ (Vis) and NIRQUEST (IR) spectrometers, it was difficult to match the transmission scales in these two wavelength regions. What is the origin of this absorption and does it appear in other minerals?
A search through my own mineral visible spectra showed that peridot (olivine) also has essentially zero transmission at 1µm. Also, the spectrum of garnet (pyrope/alamandine) is decreasing rapidly towards the IR.
The compositions of these silicate minerals are:
Iolite: (Mg_2 Al_4 Si_5 O_18) with some Mg replaced by ferrous iron (Fe^2+) and manganese, and Al by ferric iron (Fe^3+)
Peridot: (Mg, Fe)_2 SiO_4
Garnet: 3MgO/3FeO Al_2O_3 3SiO_2, Mg -> pyrope; Fe -> alamandine
I then measured the infrared transmissions of peridot (a tumble-polished fragment) and garnet (a light purple/red rough pebble) to produce these transmission and absorbance spectra. Due to the nature of the specimens, the transmission measurement have a somewhat arbitrary scale due to refraction and scattering effects and so should be considered only representative of true absorbance measurements. The relative transmission scale of the peridot in the Vis and IR is not well-determined but that of the garnet is better.
The two plots here show the result of these and the previous iolite measurements for a sample ~.8mm deep. They show the transmission vs. wavelength (in nm) and the absorbance (=-Log_10(transmission) called optical density) vs. wavenumber (with the scale reversed to keep blue on the left). The peridot has a similar - but stronger - 1µm absorption (actually centred closer to 1.1µm) stretching from ~ 700-1600nm but with very low absorption at longer wavelengths. The garnet has the characteristic complex iron spectrum in the visible but very low transmission beyond 1100nm
The common feature of these three minerals is that they are all idiochromatic (self-coloured) due to iron as part of their basic structure. Most of the absorption features are due to ferric or ferrous iron. The peridot has the 1µm feature attributed to a ligand field transition in Fe2+ (Farrell and Newnham, 1965, The American Mineralogist, 50 1972; www.minsocam.org/ammin/AM50/AM50_1972.pdf ).
The high transmission of peridot (olivine) in the IR is thought to have a significant effect on heat transfer in the Earth's mantle by providing a channel for radiative transfer.
Note that the term 'pebble' in the legend of the lower plot refers to the garnet.