Ge 116 - Analytical
Techniques in Geochemistry
George R. Rossman
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
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
spot. Below, we show simple example of how our measured Raman
spectrum might relate to other Raman spectra you may have seen.
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
For background reading, a number of websites and books provide
information on the Raman effect.
library of Raman spectra of minerals is available at http://rruff.info/
We will use this library to identify our unknowns using the program Crystal Sleuth
on the Raman instrument.
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,
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
B) We will use infrared specular
reflectance to identify minerals.
We will use an infrared
microscope to obtain polarized spectra of the feldspar in your rock
with three objectives:
determine if there is water or OH in the rock
determine if the water is in fluid inclusion, solid inclusions
(alteration products) or bound in the structure of the feldspar
estimate the quantitative amount of water in the feldspar.
If time allows, we will obtain the optical spectrum of a pyroxene in the
visible and near-infrared region
oxidation states of iron contribute
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
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
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 at least the two RRUFF databases, the SensIR
minerals and the clay mineral standards).
FTIR-- Specular Reflectance
reflectance involves reflecting light off a smooth surface (a
). 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
experimental setup, and identify
minerals using the the libraries specific to this type of spectroscopy.
Do these phases appear to be pure, or are there signs of alteration or
mixtures in the spectra?
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 on a slice of the Ge 116 rock,
and you will obtain the infrared spectrum in two (of the three
possible) polarization directions.
C. FTIR-- Microscopic
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
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 interpret your
results, use the following article to determine the water concentration
in the white feldspars in your rock.
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
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
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
Use the following
references (or others you find) to identify the dominant oxidation
state of iron that contributes to the optical spectrum:
the TA on what we're looking for in the writeup for these two weeks:
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
A. Give me a
few examples of obtained unknowns and identifications using ATR and
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: 21-Jan-2013