Ge 116 - Analytical Techniques in Geochemistry

Visible and Infrared Spectroscopies

George R. Rossman

 

Week of Jan 31 - Feb 4, 2022

 Week of Feb 7 - Feb 11, 2022


Week 1: Raman Lab  -- Room 356 Arms

You will learn to use a Raman spectrometer and will use the instrument to identify minerals in your sections.

We will need to remove the carbon coating on our polished sections to avoid getting a signal from the carbon.  We will be able to focus the Raman beam to a couple micrometers diameter. For this, it is important that you bring the SEM images (map) of your sample for guidance.

In this lab, we will turn on the Raman system and first calibrate the wavelength response (that can shift due to thermal effects) using the location of a peak in the spectrum of a silicon standard.  You will determine the measurement location by focusing with a microscope, then you will focus the laser light to the same spot.  Below, we show simple example of how our measured Raman spectrum might relate to other Raman spectra you may have seen.

Illustration of the Raman spectrum

The relationship between the Stokes and anti-Stokes transitions is dictated by the Boltzmann distribution-- in other words, some of the available vibrational modes are occupied as a function of the system's temperature. For the sake of resolution around the low-energy Raman shifts, we only measure the Stokes transitions in the current machine set-up.  As the figure illustrates, the information is the same, but we lose the ability to measure in-situ temperature. The Raman spectra are usually presented as Raman Shift in units of  wavenumbers (cm-1) from the laser line (at zero).
Quartz Raman

For background reading, a number of websites and books provide information on the Raman effect.
A library of Raman spectra of minerals is available at  http://rruff.info/ and 8s incorporated in our instrument.
We will use the library in our instrument to identify our unknowns. 

The objective of the exercise is to find and identify as many minerals as you can with the Raman instrument.  Such measurements will complement the chemical identifications made on the electron-beam instruments,



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Week 2: Infrared Lab  --- Room 354 Arms

This week, you will learn to use the infrared spectrometer (an FTIR - Fourier Transform IR).  There are a few objectives (turn these in):
A)     We will identify a unknown minerals using the ATR technique (video).
B)     We will use infrared specular reflectance to identify minerals.
C)     We will use an infrared microscope to obtain polarized spectra of the feldspar in your rock with three objectives:
            1) determine if there is water or OH in the rock
            2) determine if the water is in fluid inclusion, solid inclusions (alteration products) or bound in the structure of the feldspar
            3) estimate the quantitative amount of water in the feldspar.
D)     If time allows, we will obtain the optical spectrum of a pyroxene in the visible and near-infrared region
            1) Which oxidation states of iron contribute
            2) in which cation site is the spectroscopically dominant cation located.



A.  FTIR-ATR. Fourier Transform Infrared-- Attenuated Total Reflection

We will identify an unknown mineral with the ATR method. This can be either a small amount of material separated from your rock (but beware of mixtures) or it may be another mineral we have available in the lab or the Caltech collections.

Use a diamond or tungsten carbide point to scrape a small quantity of a single phase from your rock. Alternatively, we can give you a small piece of an 'unknown' phase.
The type of ATR assembly we use is known as a DuraScope, and has the unique ability to allow us to view and analyze our sample simultaneously. Center your sample on the ATR plate, which is a diamond window in a larger steel plate (see figure below). The ATR method requires that the sample be in complete contact with the diamond window so that the evanescent wave measures our sample, not air. This type of contact can be achieved with an applied pressure.
ATR schematic

Operate the FTIR OMNIC program using the DuraScope experimental setup. Verify that the experimental setup calls for 4 wavenumber resolution over the maximum energy range of the KBr detector (useful for general phase identification. When we look for OH, we will use the MCT detector), and that there is a viable interference pattern in the interferogram.  Collect a background spectrum, then collect infrared spectra of a few different unknowns.

Identify the unknown phase using the available libraries of mineral ATR spectra (hint-- under library setup, pick the Ge 116 database).


B.  FTIR-- Specular Reflectance

Specular reflectance involves reflecting light off a smooth surface (a specular reflection). This method is useful in that it is non-destructive and allows us to analyze minerals that are difficult to separate from their host rock.  In our case, we will use a polished slab of the rock that has individual crystals large enough to fill the ~2 mm beam of the reflectance accessory.  Spectroscopy using specular reflectance takes advantage of the fact that the reflectivity of a sample increases at the wavelength of IR absorption.  We will need to swap out the DuraScope assembly with the micro specular reflectance assembly (see below), switch our program to use the Specular Reflectance experimental setup, and identify minerals using the the libraries specific to this type of spectroscopy.

Specular

Do these phases appear to be pure, or are there signs of alteration or mixtures in the spectra?

C.  FTIR-- Microscopic Transmission

You will use infrared spectroscopy to analyze the amount of water in your feldspars. Electron microprobe and SEM-EDS analyses were unable to detect the very light elements. Hydrogen is an element that infrared can easily detect in the form of water molecules and OH groups.

We will instruct you how to do micro-infrared spectroscopy first on a garnet, then on a slice of the Ge 116 rock, and you will obtain the infrared spectrum in two (of the three possible) polarization directions.  The general routine will be to focus the microscope on a transparent sample, then focus the IR measurement area aperture on the same location, then obtain a transmission IR spectrum under the Microscope experimental setup.  The detector we use for this section is a Mercury-Cadmium-Telluride alloy (MCT), good for measuring absorptions in the mid-IR range, which includes X-OH vibrations.


To detemine the water content of the garnets, use:

American Mineralogist (1995) 80 (5-6): 465–474.

Quantitative analysis of trace OH in garnet and pyroxenes
by Bell DR, Ihinger PD, Rossman GR


To interpret your feldspar results, use the following article to determine the water concentration in the white feldspars in your rock.

American Mineralogist, Volume 88, pages 901-911, 2003

The concentration and speciation of hydrogen in feldspars using FTIR and 1H MAS NMR spectroscopy

ELIZABETH A. JOHNSON AND GEORGE R. ROSSMAN

Is the water that you see due to fluid inclusions, clays (or other alteration products) or structurally-bound water in the feldspar structure?

Ideally you would have three spectra taken in three orthogonal polarization directions to use for the determination of the total water content in the feldspar. In our case we have only have two. So, after looking at the reference above, make an educated guess about the intensity in the third direction.

Sum the integrated intensities for the three polarization directions and determine the water content of this feldspar.

Is the water content you measured in the range previously found for feldspars or is the result suspect?

D.  Optical spectra determination of iron site-occupancy  (if time allows)


We will learn to use the optical spectrometer by transmitting light through a pyroxene crystal on a thin section. We will locate the two extinction directions and obtain polarized spectra in those directions. You will obtain spectra under two different conditions. One with a visible light detector (silicon diode array) and the other with a near-infrared detector (indium gallium arsenide diode array). Merge the two spectra together in Excel or another graphics program. Be sure to pay attention to the absolute intensity and which polarization directions the spectra correspond to.  

Use the following references (or others you find) to identify the dominant oxidation state of iron that contributes to the optical spectrum:

http://articles.adsabs.harvard.edu//full/1977LPICo.304..156R/0000156.000.html

http://www.agu.org/journals/je/je0705/2006JE002802/2006JE002802.pdf

http://minerals.gps.caltech.edu/FILES/Visible/pyroxene/Index.htm





Note from the TA on what we're looking for in the writeup for these two weeks:


RAMAN.  
Give me a few examples of obtained spectra/identifications.  Combining the results from your SEM, microprobe, and now Raman analyses, what is the updated mineral list for your thin section?  Present your results in a way that shows which minerals were identified by which method, the chemistry, and the estimated volumetric proportions. If the Raman spectrum either totally fails to identify the unknown phase, or if it is unable to narrow a list of possible minerals, discuss why these problems occur.


IR/VIS.  
A.  Give me a few examples of obtained unknowns and identifications using ATR and specular reflectance.
B.  Give me representative polarized spectra, and tell me: Is there water? What kind, and how do you know? How much (with what assumptions)? Are your values similar to those previously reported?
C.  Give me the stitched spectra for both polarizations. What is the oxidation sstate of Fe, and which cation site is it dominantly located in?


Finally, or dispersed through your report, give me a quick, one sentence summary of each of these methods, including what kind of measurement or sample it is best for.  In other words, why would you use this method over your other available tools?




Last updated: 4-Jan-2022