Page 27 - Sedimentology and Stratigraphy

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9781405193795_4_0
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Nichols/Sedimentology
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
Stratigraphy
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14
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26.2.2009
26.2.2009 8:14pm Compositor Name: ARaju
Terrigenous Clastic Sediments: Gravel, Sand and Mud
14 Nichols/Sedimentology and Stratigraphy 9781405193795_4_0 02 Final Proof page 14 26.2.2009 8:14pm Compositor Name: ARaju
allows light waves vibrating in one plane to pass
through it and hence through the thin-section.
Toward the bottom of the eyepiece tube there is a
second polarising filter that is retractable. This polar-
ising filter is mounted perpendicular to the one below
the stage, such that it only allows through light
waves that are vibrating at ninety degrees to the
lower one. If this second filter, known as the analys-
ing filter, is inserted across the lenses when there is
no thin-section, or just plain glass, on the stage, then
all the light from the beam will be cut out and it
appears black. The same effect can be achieved with
‘Polaroid’ sunglasses: putting two Polaroid lenses at
ninety degrees to each other should result in the
Fig. 2.12 A photomicrograph of a sandstone: the grains are
all quartz but appear different shades of grey under crossed blocking out of all light.
polars due to different orientations of the grains. Other standard features on a petrographic micro-
scope are a set of lenses at the end of the eyepiece tube
of the rock, a very thin (normally 30 microns) slice of that allow different magnifications of viewing to be
the rock, which can be examined under a petrologi- achieved. The total magnification will be a multiple of
cal/petrographic microscope (Fig. 2.12). Thin- one of these lenses and the eyepiece magnification.
section examination is a standard technique for the The eyepiece itself has a very fine cross-wire mounted
analysis of almost all types of rock, igneous and meta- in it: this acts as a frame of reference to be used when
morphic as well as sedimentary, and the procedures the orientation of the thin-section is changed by rotat-
form part of the training of most geologists. ing the stage. The stage itself is graduated in degrees
around the edge so that the amount of rotation can
be measured. An optional feature within the eyepiece
The petrographic microscope
is a graticule, a scale that allows measurements of
A thin-section of a rock is cemented onto a glass features of the thin-section to be made if the magnifi-
microscope slide and it is normal practice to cement cation is known.
a thin glass cover slip over the top of the rock slice to There are usually further tools for optical analyses
form a sandwich, but there are circumstances where on the microscope, such as additional lenses that can
the thin-section is left uncovered (3.1.2). The slide is be inserted above and below the stage, and plates that
placed on the microscope stage where a beam of white can be introduced into the eyepiece tube. These are
light is projected through the slide and up through the used when advanced petrographic techniques are
lenses to the eyepiece: this transmitted light micro- employed to make more detailed analyses of minerals.
scopy is the normal technique for the examination of However, at an introductory level of sedimentary pet-
rocks, the main exceptions being ore minerals, which rography, such techniques are rarely used, and anal-
are examined using reflected light (this is because of ysis can be carried out using only a limited range of
the optical properties of the minerals concerned – see the optical properties of minerals, which are described
below). The majority of minerals are translucent in the following sections.
when they are sliced to 30 microns thick, whatever
their colour or appearance in hand specimen: this is
particularly true of silicate and carbonate minerals, 2.3.5 Thin-section analysis of sandstones
which are the groups of prime interest to the sedimen-
tary geologist. It is therefore possible to view the Use of the following techniques will allow identifica-
optical properties of the minerals, the way they tion of the most frequently encountered minerals in
appear and interact with the light going through sedimentary rocks. Only a very basic introduction to
them, using a petrographic microscope. the principles and application of thin-section analysis
Underneath the microscope stage the light beam is provided here. For more detailed and advanced
passes through a polarising filter, which only petrographic analysis, reference should be made to