Geology Reference
In-Depth Information
is the splitting of d energy levels in transition-metal ions
such as iron (Box 9.11). Elements like iron or manganese,
if present in a mineral, play a major part in determining
its colour. This can be seen in a solid solution series like
phlogopite-biotite. The Mg-rich mica phlogopite
(KMg 3 {AlSi 3 O 10 }[OH] 2 ) is colourless or very pale in thin
section, whereas the iron end-member biotite is intensely
brown or red. Colour attributable to an essential con-
stituent is said to be idiochromatic.
Certain other minerals, including some important
gemstones, owe their colour to impurity elements. A
common agent of such allochromatic colours is tri-
valent chromium, responsible for the deep red colour
of ruby (a variety of corundum, Al 2 O 3 ) and the green of
emerald (a variety of beryl). The 'chrome diopside'
which is conspicuous in many mantle-derived perido-
tites also owes its brilliant green colour to its Cr 2 O 3
content (1% or so).
Colour can also be due to the presence of impurity
phases . Quartz sometimes takes on a blue colour due to
the presence of microscopic needles of rutile (TiO 2 ) or
Minerals may become coloured from being exposed
to natural radiation for long periods. Such colours are
due to radiation-induced defects in the crystal struc-
ture. The purple colour of fluorite, for example, is
attributed to free electrons occupying vacant anion
sites called 'colour centres'. An electron trapped in
such a vacancy, like an atomic electron, is restricted to
a number of quantized energy levels and transitions
between these levels generate the observed colours.
Heating eliminates most of the vacancies in a crystal
and therefore bleaches the colour.
Metals and a number of important minerals - nota-
bly graphite and certain sulfides - absorb all light pass-
ing through them, so that in thin section they transmit
no light at all. The property of opacity can be traced to
metallic or quasi-metallic bonding in the crystal.
Delocalized electrons in a metal behave like a charged
fluid (Chapter  7). The alternating polarization of this
electron fluid, induced by the incoming light's oscill-
ating electric field, involves work being done in mov-
ing electrons around, and this dissipates the energy of
the incident light (transforming it into thermal motion)
just as the energy of a walker is sapped by walking
across unconsolidated dry sand. A seismological anal-
ogy is the absorption of shear waves by liquids, which
is why such waves do not pass through the Earth's
core whereas compressional waves do. Insulating
materials like quartz, in which there are no delocalized
electrons, are electromagnetically elastic: electrons
fixed in atoms cannot be moved around and so absorb
practically no energy from the beam, which is there-
fore transmitted with little loss of intensity like seismic
waves through solid rock.
The striking characteristic of minerals like pyrite and
galena is their metal-like reflectivity . A reflected-light
microscope with a special reflectivity accessory can be
used to measure quantitatively the proportion of the
perpendicularly incident light that is reflected back at
various wavelengths, and this provides a vital diagnostic
tool for the ore mineralogist. The reflectivity of a mineral
increases with its refractive index and with its opacity.
Whereas minerals like halite and diamond have opt-
ical properties that are uniform in all directions, the
majority of minerals are optically anisotropic . That is
to say, the refractive index, the colour and - for ore
minerals - the reflectivity vary according to the direc-
tion in which the incoming light (specifically its elec-
tric vector) oscillates.
Incoming light entering an anisotropic mineral is
split into two rays with mutually perpendicular vibra-
tion directions, which experience different refractive
indices as they travel through the crystal. The birefrin-
gence of the crystal is the difference between its maxi-
mum and minimum refractive indices. The mineral
calcite is among the most birefringent of the common
minerals, to the extent that the phenomenon can even
be seen without the aid of a microscope. To understand
why this is, we need to note that the carbonate anion
CO 3 2− has a symmetrical planar shape (Figure  8.5),
reflecting the geometry of the sp 2 -hybrid that the car-
bon atom uses to form σ -bonds with the three oxygen
atoms. All the CO 3 2− ions in calcite have the same orien-
tation perpendicular to the three-fold symmetry
( z ) axis of the crystal. The remaining carbon p-electron
can establish a π -bond with any one of the three oxy-
gens, giving three possible configurations. As with
graphite (Box 7.4), the real configuration is a blend of
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