EL NEOLIBERALISMO Y EL MEDIO AMBIENTE

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.