Peak Position, cm * Intensity. Width, cm * Integral.
8929 03 9 3207 1330
11220 0.60 2673 1926
13378 0.41 2956 1281
15567 0.31 3517 1162
This would mean that the crystal field transitions would have very similar
energies. Given the widths o f the two bands, (3207cm*^ and 2672cm \ respectively),
it is likely that there are a number o f crystal field bands superimposed on one another,
arising fi-om the 'Tjg transition o f Fe^^ in all three o f the cation sites, and that
the curve fitting program cannot resolve the other peaks. This was found to be a
problem by Keppler et al. (1995) in their study o f (N%, Fe)Si03 perovskite.
An accurate energy level diagram for any o f the three cation sites would be
hard to work out because o f this superposition o f peaks, and, therefore no attenpt has
been made to derive one. However, some idea o f the crystal field parameters can be
obtained if two assumptions are made. The first assumption is that the two curves
fitted to the spectrum can be taken as averages o f the peaks due to the crystal field
transitions. From this, the position o f the baricentre o f the eg orbitals can be obtained.
For the single crystal o f wadsleyite at OGPa, the position o f the baricentre is 10075cm'
\ The second assumption concerns the sphtting o f the tzg orbitals. From Table 6.1, it can be seen that for a wadsleyite with 16% Fe, the quadratic elongation parameter
ranges fi*om 1.0053 to 1.0066. As a comparison, the quadratic elongation o f the cation
site in ringwoodite is 1.0005 (Bums, 1993). Bums and Sung (1978) suggested that the
best estimate for the lower level sphtting in ringwoodite is lOOOcm'^ and given the
similarity in the distortions o f the sites in the wadsleyite and ringwoodite structures,
this is proposed to be a reasonable value for the sphtting in wadsleyite.
From this, a value for \ can be obtained, and for wadsleyite at OGPa, the octahedral sphtting is equal to 9575cm'\ This value can then be used to obtain the
CFSE o f the phase, as:
CFSE=Q . A A ^ +o (6.1)
wiiere a is equal to half the splitting between the orbitals. For the wadsleyite at
OGPa, the CFSE equals 4330cm \ In comparison, the values o f and CFSE for
fayalite olivine are 8883cm'^ and 4230cm'^ respectively for the M l site, and 8553cm'^
and 4158cm^ respectively for the M2 site, yielding average values o f 8718cm ^ for \
and 4194cm ^ for the CFSE. Ringwoodite has values for the crystal field parameters
o f 10763cm'^ and 4972cm'^ for \ and the CFSE. The values for the crystal field parameters are therefore much closer to those o f the fayalite than the ringwoodite.
The two remaining bands in the spectrum, at 13378cm'^ and 15567cm \ are
attributed to Fe^^ -^Fe^^ charge transfer, in agreement with the presence o f Fe^^^ in
the Mossbauer data wiiich is indicative o f IVCT. The widths o f the bands are also
characteristic o f IVCT transitions (Mattson and Rossman, 1987) and the position o f
the bands are close to those for other Fe^^ Fe^^ transitions in minerals containing
edge sharing octahedra (Table 6.3 and Bums, 1993). With the low concentration o f
FeP^ in the sample, the IVCT is likely to be o f type llIA, according to the
classification o f Day ( 1976) with the nature o f the sites involved in the charge transfer
being almost identical, finite clusters o f octahedra. The fact that more than one band
is seen is due to the nature o f the structure o f wadsleyite, which consists o f network
o f edge - sharing octahedra (Fig 6.1). The chains link in three ways: there are chains
o f M l - M2 octahedra parallel to b, double chains o f M3 octahedra parallel to a, and zig - zag chains involving all three octahedra running parallel to b and c. It is the charge transfer that causes the blue - green colour o f wadsleyite.
Mineral. Metal - metal distance, pm IVCr energy, cm'^ wadsleyite* M l - NG = 287; M l - M3 = 290 13378 M2 - M3 = 305; M3 - M3 = 280; 285 15567 wadsleyite^ 13374 15921 vivianite 285 15870 babingtonite 337 14710 andalusite 265 8 10900 290.1 13400 yoderite 290 13800 oithopyroxene M l - Ml = 315 M l - M2 = 308; 327 14500 calculated 2917 10570 o
Table 6.3: Coinpaiison o f ->Fe^^ charge üansfer eneigies. * denotes this study, ^ the work o f Ross (1996). All other data horn Bums (1993).
Polarised spectra were also obtained from single crystal sanples o f wadsleyite.
Fig. 6.4 shows a spectrum polarised parallel to the a - axis. A minhnum o f four curves
were required to fit this spectrum, as shown in Fig. 6.4, and the parameters for these
curves are given in Table 6.4. Assignments for these curves follow the arguments
given for the unpolarised spectrum, with the peaks at 8282 and 10370cm*^ being due
to the crystal field transitions o f Fe^^ in the M l, M2 and M3 sites in the structure, and
the peaks at 13372 and 16137cm^ being attributed to IVCT. These IV C I bands are
due to the double chains o f M3 octahedra
A spectrum polarised parallel to the b - axis is shown in Fig. 6.5. Again, a
minimum o f four curves were required to fit this spectrum, Miich is different to that
shown in Fig. 6.4. The curve fitting parameters are given in Table 6.5. Again, the
bands assignments follow that o f the unpolarised spectrum, with the two bands below
12000cm^ being crystal field bands o f Fe^^ and the bands at 13291 and 15187cm^
being due to the charge transfer processes involving the M l - M2 octahedral chains
and the ’zig - zag' chains.
Finally, Fig. 6.6 shows a spectrum polarised parallel to c - axis. Again, four
curves were fitted to the spectrum, and the parameters are given in Table 6.6. Hie
spectrum itself is slightly broader than the other polarised spectra, and one o f the
charge transfer peaks, lying at 14113cm'^ is exceptionally weak in comparison to the
other charge transfer peaks recorded above. This is due to there being only one form
o f charge transfer along the c - axis, involving the 'zig - zag' chains as described
above.
What is noticeable from all the polarised spectra is that there is charge transfer
oo <u