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Mineral Spectroscopy Visible Infrared Mössbauer Raman NMR Properties of Light • Light is conducted through materials on the valence electrons. • Light travels more slowly in materials. • Electrically conducting materials are opaque Electromagnetic Spectrum Visible Light: 7700 - 3900Å Properties of Light • Light is conducted through materials on the valence electrons. • Light travels more slowly in materials. • Electrically conducting materials are opaque. Behavior of Light in Materials • Absorption (light is absorbed by materials) • Color (absorption is a function of wavelength) • Pleochroism (absorption is a function of direction) • Refraction (light travels slowly in some materials) • Dispersion (velocity is a function of wavelength) • Birefringence (velocity is a function of direction) Absorption • Light is attenuated on entering any • • • • • material. The attenuation is a function of distance. I is intensity at some point t I0 is initial intensity. k is absorption coefficient in cm-1. Lambert’s Law: I / I0 e kt Color • Absorption may be a function of wavelength. • Materials may appear colored in transmitted light Color • Absorption in the visible mainly due to electron transitions in d-orbitals or f-orbitals. • Color in minerals primarily due to presence of transition metals or rare earth elements. Visible and NIR spectra of Ametrine (quartz) Courtesy: George Rossman Ringwoodite is Blue • (g-Mg1.63Fe0.22 H0.4 Si0.95O4) • ~10 % of Fe present as ferric (Mössbauer) Pleochroism • Pleochroism is the variation of absorption with direction in a crystal. • Pleochroism is observed as a color change on rotation in plane-polarized light (not crossed polars). • Pleochroism only occurs in non-cubic crystals. • Pleochroism indicates the presence of transition metals (esp Fe, also Mn, Cr, V, etc). • Biotite, tourmaline, amphibole. Refraction and Reflection • When light strikes a polished surface of a material it is split into two rays. • One is reflected and the other refracted Infrared spectroscopy • Near IR 5000 - 13000cm-1 – orbital transitions • Mid-IR 2500 - 5000cm-1 – N-H and O-H bond vibrations • Far IR 500 - 2500 cm-1 – Cation-Oxygen bond vibrations – Structural phonons. FTIR Spectrometer Mid IR spectroscopy Mid IR spectroscopy Raman Spectroscopy • Looks at wavelength shifts in scattered light. • Shifts are in atomic vibrational part of spectrum • 0 - 5000cm-1. (same as mid to far IR) • Excitation is usually by a monochromatic source in the visible region (commonly a laser). Raman Spectroscopy Mössbauer Spectroscopy Resonant Gamma Ray spectroscopy Uses 57Fe gamma decay at 14.4 MeV Source is 57Co Source is accelerated mechanically to produce ultra-fine relativistic energy shifts • Absorption as a function of source velocity • Looks at electric field effects at nucleus due to d-orbital occupancy and perturbations from local coordination effects • • • • Mössbauer spectroscopy Mössbauer spectroscopy Mössbauer spectroscopy NMR Spectroscopy • Nuclear Magnetic Resonance • Similar to Mössbauer spectroscopy but • • • • many more nuclides Radio frequency emission spectroscopy due to magnetic transitions in nucleus. Solid samples are spun in a strong magnetic field (Magic Angle Spinning) A RF field applied and turned off. Sample emits RF NMR Spectroscopy NMR Spectroscopy NMR Spectroscopy