Page 31 - Sedimentology and Stratigraphy

9781405193795_4_0
9781405193795_4_0
Final Proof page 18
Name:
ARaju
Stratigraphy
and
Stratigraphy
02
02
Compositor
Final
Proof
Nichols/Sedimentology
and
Nichols/Sedimentology
26.2.2009
8:14pm
26.2.2009 8:14pm Compositor Name: ARaju
page
18
Terrigenous Clastic Sediments: Gravel, Sand and Mud
18 Nichols/Sedimentology and Stratigraphy 9781405193795_4_002 Final Proof page 18 26.2.2009 8:14pm Compositor Name: ARaju
their composition: albite is the sodium-rich form, and grain in a sandstone. This is because of the suscep-
anorthite the calcium-rich, with several others in tibility of these silicate minerals to chemical break-
between. The most characteristic distinguishing fea- down at the Earth’s surface, and they do not generally
ture is the occurrence of multiple twins, which give survive for long enough to be incorporated into a
the grains a very pronounced black and white striped sediment.
appearance under crossed polars. The extinction
angle varies with the composition, and is used as a Glauconite
way of distinguishing different minerals in the plagi-
oclase group (Gribble & Hall 1999; Nesse 2004). This distinctive green mineral is unusual because,
unlike other silicates, it does not originate from
igneous or metamorphic sources. It forms in sediment
Micas
on the sea floor and can accumulate to form signifi-
There are many varieties of mica, but two of the most cant proportions of some shallow marine deposits
frequently encountered forms are the white mica, (11.5.1). Under plane-polarised light glauconite
muscovite, and the brown mica, biotite. Micas are grains have a distinctive, strong green colour that is
phyllosilicates, that is, they have a crystal structure patchy and uneven over the area of the grain: this
of thin sheets, and have a very well developed platy colour mottling is because the mineral normally
cleavage that causes the crystals to break up into very occurs in an amorphous form, and other crystal prop-
thin grains. If the platy grains lie parallel to the plane erties are rarely seen.
of the thin-section, they will appear hexagonal, but it
is much more common to encounter grains that have
Carbonate minerals
been cut oblique to this and therefore show the clea-
vage very clearly in thin-section. The grains also The most common minerals in this group are the
appear elongate and may be bent: mica flakes are calcium carbonates, calcite and aragonite, while dolo-
quite delicate and can get squeezed between harder mite (a magnesium–calcium carbonate) and siderite
grains when a sandstone is compacted (18.3.1). Bio- (iron carbonate) are also frequently encountered in
tite is usually very distinctive because of its shape, sedimentary rocks. Calcium carbonate minerals are
cleavage, brown colour and pleochroism (which extremely common in sedimentary rocks, being the
may not always be present). It has bright, first-order main constituents of limestone. Calcite and aragonite
birefringence colours, but these are often masked by are indistinguishable in thin-section: like all sedimen-
the brown mineral colour: the extinction angle is 08 tary carbonates, these minerals have a high relief and
to 38. The strong, bright birefringence colours of crystals show two clear cleavage planes present at 758
muscovite flakes are very striking under cross-polars, to each other. Birefringence colours are pale, high-
which along with the elongate shape and cleavage order greens and pinks. The form of calcite in a sedi-
make this a distinctive mineral. mentary rock varies considerably because much of it
has a biogenic origin: the recognition of carbonate
components in thin-section is considered in section
Other silicate minerals
3.1.2.
In comparison to igneous rocks, sedimentary rocks Most dolomite is a diagenetic product (18.4.2), the
contain a much smaller range of silicate minerals as result of alteration of a limestone that was originally
common components. Whereas minerals belonging to composed of calcium carbonate minerals. When indi-
the amphibole, pyroxene and olivine groups are vidual crystals can be seen they have a distinctive
essential minerals in igneous rocks of intermediate euhedral rhombic shape, and cleavage planes parallel
to mafic composition (i.e. containing moderate to to the crystal faces may be evident. The euhedral
relatively low proportions of SiO 2 ), these minerals morphology can be a good clue, but identification of
are rare in sediments. Hornblende, an amphibole, is dolomite cannot be confirmed without chemical tests
the most frequently encountered, but would normally on the material (3.1.2). Siderite is very difficult to
be considered a ‘heavy mineral’ (see below), as would distinguish from calcite because most of its optical
any minerals of the pyroxene group. Olivine, so com- properties are identical. The best clue is often a slight
mon in gabbros and basalts, is very rare as a detrital yellow or brownish tinge to the grain, which is a