Mineral Spectroscopy Visible Infrared Raman Mössbauer NMR

Properties of Light Light is conducted through materials on the valence electrons. Light travels more slowly in materials. Electrically conducting materials are opaque

Visible Light: Å Electromagnetic Spectrum

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 I 0 is initial intensity. k is absorption coefficient in cm -1. Lambert’s Law:

ColorColor Absorption may be a function of wavelength. Materials may appear colored in transmitted light

ColorColor 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  -Mg Fe 0.22 H 0.4 Si 0.95 O 4 ) ~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 cm -1 –orbital transitions Mid-IR cm -1 –N-H and O-H bond vibrations Far IR cm -1 –Cation-Oxygen bond vibrations –Structural phonons.

FTIR Spectrometer

Mid IR spectroscopy

Raman Spectroscopy Looks at wavelength shifts in scattered light. Shifts are in atomic vibrational part of spectrum cm -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 57 Fe gamma decay at 14.4 MeV Source is 57 Co 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

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