Geology Reference
In-Depth Information
Highly polarising
6
S
n +
5
N
P
Nb Ta
4
C
Si
Ti
Sn Zr Hf
U Th
REE
GaV Sc
Cr Fe
Al iv Al vi
3
B
Lu Y La
Mn Mg Co Fe
Ni Cu Zn
2
Be
Ca
Eu Sr Pb
Ba
1
Li
Na
K
Rb Cs
1.0 Å
2.0 Å
0
0.00
0.05
0.10
0.15
0.20
r
Ionic radius/nm
Figure 7.7 Ionic potentials of common 'cations'. 'REE' refers to the rare earth elements La ( Z = 57) to Lu ( Z = 71) - see
Chapter 6. Yttrium (Y) has similar properties.
(an analytical technique called 'neutron activation
analysis'). Silicon is used in a similar way as an X-ray
detector in the electron microprobe (Box 6.4). A Ge (or
Si) crystal, even with several hundred DC volts applied
across it, will conduct no appreciable current 3 except
when it absorbs a γ -ray (or X-ray) photon. In either case
the photon energy is sufficient to promote a number of
electrons temporarily into the conduction band, leav-
ing an equivalent number of 'holes' in the valence
band. The arrival of the photon is thus marked by a
short pulse of electric current through the crystal, after
which it relaxes back to its insulating condition. As the
number of electrons promoted is proportional to the
photon energy, the amplitude of the output pulse will
vary in proportion to the quantum energy of the photon
detected. Electronically sorting the pulses according to
pulse height therefore provides a simple and econom-
ical means of isolating the various components of a
complex γ -ray (or X-ray) spectrum; this is the basis of
energy-dispersive ('ED') spectrometry (Box  6.4). The
intensity of each spectral 'line' is obtained by counting
the number of pulses per second that are recorded
within the appropriate pulse-height interval.
The energy gap between filled and conduction bands
in a semiconductor can be deliberately engineered for
specific applications by 'doping' the crystal surface
with other elements, notably those of Groups IIIb and
Vb; the doping elements introduce additional energy
levels into the gap.
Bonding in minerals
We have seen that ionic bonding develops between
elements of very different electronegativity, whereas
covalent bonds are characteristic of materials, includ-
ing pure elements, in which the electronegativity con-
trast is slight. Most of the compounds we meet as
minerals fall between these two extremes, however, so
we now have to consider what sort of chemical bond-
ing operates between elements that show moderate
differences in electronegativity. We shall investigate
this intermediate domain by considering how real
bonds deviate from the idealized ionic and covalent
models.
To prevent thermal excitation of electrons into the conduction
band creating a spurious signal, Ge and Si detectors are oper-
ated at low temperatures maintained by thermal contact with
a liquid nitrogen reservoir (−196 °C).
3
 
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