Page 29 - Sedimentology and Stratigraphy

Compositor
Name:
8:14pm
Nichols/Sedimentology
Nichols/Sedimentology
16
page
26.2.2009
ARaju
26.2.2009 8:14pm Compositor Name: ARaju
and
02
02
Final
Proof
Final Proof page 16
Stratigraphy
and
Stratigraphy
9781405193795_4_0
9781405193795_4_0
Terrigenous Clastic Sediments: Gravel, Sand and Mud
16 Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 16 26.2.2009 8:14pm Compositor Name: ARaju
other when the grain is moved by rotating the Angle of extinction
microscope stage. This phenomenon is known as
When the stage is rotated, the birefringence colour of
pleochroism and is also seen in biotite mica and a
a grain of an anisotropic mineral will vary as the
number of other minerals. It is caused by variations
crystal orientation is rotated with respect to the
in the degree of absorption of different wavelengths
plane-polarised light. The grain will pass through a
of light when the crystal lattice is at different
‘maximum’ colour (although this may not be the
orientations.
maximum colour for this mineral, as this will depend
on the three-dimensional orientation of the grain) and
Birefringence colours will pass through a point in the rotation when the
grain is dark: this occurs when the crystal lattice is in
When the analysing lens is inserted across the objec- an orientation when it does not influence the path of
tive/eyepiece tube, the appearance of the minerals in the polarised light. With some minerals the grain goes
the thin-section changes dramatically. Grains that black – goes into extinction – when the grain is
had appeared colourless under plane-polarised light oriented with the plane of the polarised light parallel
take on a range of colours, black, white or shades of to a crystal face: this is referred to as parallel extinc-
grey, and this is a consequence of the way the tion. When viewed through the eyepiece of the micro-
polarised light has interacted with the minerals.
scope the grain will go into extinction when the
Non-opaque minerals can be divided into two groups:
crystal face is parallel to the vertical cross-wire.
isotropic minerals have crystal lattices that do not
Many mineral types go into extinction at an angle to
have any effect on the pathway of light passing
the plane of the polarised light: this can be measured
through them, whatever orientation they are in
by rotating a grain that has a crystal face parallel to
(halite is an example of an isotropic mineral); when
the vertical cross-wire until it goes into extinction and
light passes through a crystal of an anisotropic
measuring the angle against a reference point on the
mineral, the pathway of the light is modified, and
edge of the circular stage. Different types of feldspar
the degree to which it is affected depends on the
can be distinguished on the basis of their extinction
orientation of the crystal. When a crystal of an iso-
angle.
tropic mineral is viewed with both the polarising and
analysing filters inserted (under cross-polars), it
appears black. However, an anisotropic mineral will Twinning of crystals
distort the light passing through it, and some of the
light passes through the analyser. The mineral will Certain minerals commonly display a phenomenon
then appear to have a colour, a birefringence known as twinning, when two crystals have formed
colour, which will vary in hue and intensity depend- adjacent to each other but with opposite orientations
ing on the mineral type and the orientation of the of the crystal lattice (i.e. mirror images). Twinned
particular grain (and, in fact, the thickness of the crystals may be difficult to recognise under plane-
slice, but thin-sections are normally cut to 30 polarised light, but when viewed under crossed polars
microns, so this is not usually a consideration). the two crystals will go into extinction at 1808 to each
For any given mineral type there will be a ‘max- other. Multiple twins may also occur, and in fact are a
imum’ birefringence colour on a spectrum of colours characteristic of plagioclase feldspars, and these are
and hues that can be illustrated on a birefringence seen as having a distinctive striped appearance under
chart. In a general sense, minerals can be described as crossed polars.
having one of the following: ‘low’ birefringence col-
ours, which are greys (quartz and feldspars are exam-
ples), ‘first order’ colours (seen in micas), which are 2.3.6 The commonest minerals in
quite intense colours of the rainbow, and ‘high order’ sedimentary rocks
colours, which are pale pinks and greens (common in
carbonate minerals). Petrology reference books (e.g. Almost any mineral which is stable under surface
Gribble & Hall 1999; Nesse 2004) include charts conditions could occur as a detrital grain in a sedi-
that show the birefringence colours for common mentary rock. In practice, however, a relatively small
minerals. number of minerals constitute the vast majority of