4.2.1: Visible and near infi-ared spectra.
Ideally, the easiest way to obtain a spectrum fi*om a crystalline sanple is to cut
a plate o f the sample using an annular saw, and doubly polish the plate down to
around 1mm thickness, which is a standard thickness used for visible to near infi^ared
work. The polishing process is done on a proprietary polishing machine, which
consists o f a revolving metal plate, and a weighted oscillating arm The sanple is
polished using finer and finer grades o f diamond paste until the sarrple has reached
the desired thickness, and then further polished to produce an even surface fi*ee o f
polishing marks. To prevent the sanple only being polished in one direction, the
sample holder can fi’eely rotate on the end o f the arm
The above method also holds tme for single crystals fi'om multi - anvil press
charges. However, if the sample is a powder, or is a mass o f small crystallites, which
can sometimes be produced in multi - anvil press experiments, then a different method
must be used. The sample is first ground in a pestle and mortar to ensure the particles
are about the same size, then mixed with an amount o f dry KBr, to dilute the sanple.
This is normally done in the ratio 200 parts KBr to one part sample. An amount o f
this KBr / sample mixture is then pressed into a thin pellet, (ideally about 1mm thick),
in a vacuum, to ensure the KBr is kept dry. The pellet is then kept in an oven
overnight at 110°C, and repressed the next day, just before being placed in the sartple
chamber in a suitable holder. If done correctly, the pellet is translucent, and enough
light can pass through it to record a spectrum from the particles o f sample in the
pellet. This method is usually used for studies in the mid infrared, as KBr is
transparent to mid infrared radiation.
4.2.2: Diffuse Reflectance Spectroscopy.
During the course o f this study, it was found that for visible and near infrared
studies o f fine grained samples (most noticeably the wadsleyite), that the KBr pellet
method did not work as well as was hoped For some o f these, more normally those
samples consisting o f crystallites, thin films were prepared by placing the sanple in
an ungasketed diamond anvil cell. For the ferropericlase study, this was not a suitable
method, and diffuse reflectance spectra were recorded instead
This is a method usually applied to regoliths on planets, allowing some idea
o f the mineralogy o f the soil to be worked out from the spectra obtained For a
polycrystalline assemblage, the reflectance from the surface, R j is the sum o f the
surface reflectance, R g, and the volume reflectance, Ry (sometimes called the diffuse
reflectance). The surface reflectance is that part o f the total reflectance that has not
penetrated any o f the particles, whereas the volume reflectance is that component
which has been transmitted through one or more particles.
If the absorption coefficient o f the material is large, or the size o f the particles
is large then Rj ~ Rg . If however, the particle size and absorption coefficients are
small, the % Ry , and the Kebidka - Munk theory o f difilise reflectance becomes
applicable.
In difilise reflectance spectra, the positions o f absorption bands measured by
normal absorption spectroscopy correspond to areas o f minimum reflectance. However,
the resolution o f reflectance spectroscopy is poor, and bands are much broader and
band intensity contrasts are lost.
Comparisons between reflectance spectra and absorption spectra are made
easier by the apphcation o f the Kebulka - Munk fimctioo, which transforms the
reflectance spectrum into an absorbance spectrum The Kebulka - M ink fiinction takes
one o f two forms. I f the incident light is monochromatic, and the scattering process
shows no wavelength dependence, then the function is defined by:
(4-1)
where R^o is the reflectance from an infinitely thick sample, (normally a few
millimetres for sihcate and oxide minerals), and a and S are the absorption and
scattering coefiBcients respectively. I f however, there is some wavelength dependence
to the scattering, because the particles are cortparable in size to the wavelength o f the
radiation, then the equation becomes:
f { R J = (4.2)
wheie C is a constant, X is the wavelength o f the incident radiation and n taking values between 1 and 4.
Diffuse reflectance spectra are measured using an accessory attached to or
placed in the spectrometer. The accessory used in the experiments performed on
ferropericlase, Wiich is placed in the sample chamber, uses a pair o f mirrors to steer
the beam o f radiation onto the surface o f an integrating 'sphere', which difihises the
light. This light is reflected by the powdered sanple back onto the other side o f the
sphere, and o ff two mirrors back into the spectrometer. The reference spectrum for this
system uses an aluminised square o f emery paper, o f about 1cm in both dimensions,
and the sarrple spectrum is obtained by placing the powdered sanple on a similar
piece o f coated emery paper. Plate 4.1 shows the accessory in place in the sanple
chamber o f the spectrometer.
4.2.3: High pressure experiments.
All high pressure spectroscopic experiments were performed using diamond
anvil cells made by Diacell products o f Leicester, and are o f the lever arm type
developed by the National Bureau o f Standards laboratory (Weir et al, 1959).
The principle behind diamond anvil cells is very simple. Two opposed diamond
anvils are used, each with the culet face poUshed o ff to produce a working surface
which the sample is placed on. The two diamonds, o f around 1/3 o f a carat, are
positioned with the culet faces parallel, and the sanple is subjected to pressure when
the two diamonds are pushed together (Fig. 4.2)
Fig. 4.3 shows a sectional view o f a lever arm cell as used by Piermarini and
Block (1975) which works identically to the cells used in this study. The two
diamonds are mounted in a piston and cylinder arrangement, with one o f these being
held stationary by the cell, whilst the other is pushed upon by the lever arm, which
s
Plate 4.1: View o f the diffuse retlectance accessor}/ in place in the sample chamber ot the spectrometer at University College London.
jSample.
Ruby. Gasket.