Page 28 - Sedimentology and Stratigraphy

and
Nichols/Sedimentology
Nichols/Sedimentology
and
9781405193795_4_0
Stratigraphy
Stratigraphy
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
Compositor
Compositor
8:14pm
ARaju
Name:
Name:
ARaju
9781405193795_4_0
15
15
page
26.2.2009
8:14pm
26.2.2009 8:14pm Compositor Name: ARaju
26.2.2009
page
Final Proof page 15
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02
Final
Proof
Proof
Final
Sand and Sandstone 15
an appropriate book on optical mineralogy (e.g. Grib- useful distinguishing feature. Quartz, for example,
ble & Hall 1999; Nesse 2004), which should be used lacks a cleavage, but feldspars, which otherwise
in conjunction with suitable reference books on sedi- have many optical properties that are similar to
mentary petrography, particularly colour guides such quartz, commonly show clear, parallel lines of clea-
as Adams et al. (1984). vage planes. However, the orientation of the mineral
in the thin-section will have an important effect
because if the cut is parallel to the cleavage planes it
Grain shape
will appear as if the mineral does not have a cleavage.
A distinctive shape can be a characterising feature of The angle between pairs of cleavage planes can be
a mineral, for example members of the mica family, important distinguishing features (e.g. between
which usually appear long and thin if they have been minerals of the pyroxene family and the amphibole
cut perpendicular to their platy form. Minerals may group of minerals). The cleavage is usually best seen
also be elongate, needle-like or equant, but in all cases under plane-polarised light and often becomes clearer
it must be remembered that the shape depends on the if the intensity of the light shining through is reduced.
angle of the cut through the grain. Grain shape also
provides information about the history of the sedi- Colour and opacity
ment (2.5.4) so it is important to distinguish between
grains that show crystal faces and those that show This property is assessed using plane-polarised light
evidence of abrasion of the edges. (i.e. without the analysing filter inserted). Some miner-
als are completely clear while others appear slightly
cloudy, but are essentially still colourless: minerals
Relief
that display distinct colours in hand specimen do not
Relief is a measure of how strong the lines that mark necessarily show any colours in thin-section (e.g. pur-
the edges of the mineral, or minerals that comprise a plequartzorpinkfeldspar).Coloursmaybefainttintsor
grain, are and how clearly the grain stands out much stronger hues, the most common being shades of
against the glass or the other grains around it. It is a green and brown (some amphiboles and micas), with
visual appraisal of the refractive index of the mineral, rarer yellows and blues. (A note of caution: if a rock is
which is in turn related to its density. A mineral such rather poorly lithified, part of the process of manufac-
as quartz has a refractive index that is essentially the ture of the thin-section is to inject a resin into the pore
same as glass, so a grain of quartz 30 microns thick spaces between the grains to consolidate it; this resin is
mounted on a microscope slide will only just be visible commonly dyed bright blue so that it can easily be
(the mounting medium – glue – normally has the distinguished from the original components of the
same optical properties as the glass slide): it is there- rock – it is not a blue mineral!)
fore considered to have ‘low relief’. In contrast, a Some grains may appear black or very dark brown.
grain of calcite against glass will appear to have very The black grains are opaque minerals that do not
distinct, dark edges, because it is a denser mineral allow any light through them even when cut to a
with a higher refractive index and therefore has a thin slice. Oxides and sulphides are the commonest
‘high relief’. Because a sedimentary grain will often opaque minerals in sedimentary rocks, particularly
be surrounded by a cement (18.2.2) the contrast with iron oxides (such as haematite) and iron sulphide
the cement is important, and a quartz grain will stand (pyrite), although others may occur. Black grains
out very clearly if surrounded by a calcite cement. that have a brown edge, or grains that are dark
Certain ‘heavy minerals’, such as zircon, can readily brown throughout, are likely to be fragments of
be distinguished by their extremely high relief. organic material.
Cleavage Pleochroism
Not all minerals have a regular cleavage, a preferred A grain of hornblende, a relatively common member
fracture orientation determined by the crystal lattice of the amphibole group, may appear green or brown
structure, so the presence or absence of a cleavage when viewed under plane-polarised light, but what
when the mineral is viewed in thin-section can be a is distinctive is that it changes from one colour to the