Page 24 - Sedimentology and Stratigraphy

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Nichols/Sedimentology
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Stratigraphy
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26.2.2009 8:14pm Compositor Name: ARaju
26.2.2009
Sand and Sandstone 11
Quartz bedding planes and often have a larger surface area
than the other detrital grains in the sediment; this is
Quartz is the commonest mineral species found as because a platy grain has a lower settling velocity
grains in sandstone and siltstone. As a primary than an equant mineral grain of the same mass and
mineral it is a major constituent of granitic rocks, volume so micas stay in temporary suspension longer
occurs in some igneous rocks of intermediate compo-
than quartz or feldspar grains of the same mass.
sition and is absent from basic igneous rock types.
Metamorphic rocks such as gneisses formed from
granitic material and many coarse-grained metasedi- Heavy minerals
mentary rocks contain a high proportion of quartz.
The common minerals found in sands have densities
Quartz also occurs in veins, precipitated by hot fluids 3
of around 2.6 or 2.7 g cm : quartz has a density of
associated with igneous and metamorphic processes. 3
2.65 g cm , for example. Most sandstones contain a
Quartz is a very stable mineral that is resistant to
small proportion, commonly less than 1%, of minerals
chemical breakdown at the Earth’s surface. Grains of
that have a greater density. These heavy minerals
quartz may be broken or abraded during transport but 3
have densities greater than 2.85 g cm and are tra-
with a hardness of 7 on Mohs’ scale of hardness,
ditionally separated from the bulk of the lighter
quartz grains remain intact over long distances and
minerals by using a liquid of that density which the
long periods of transport. In hand specimen quartz
common minerals will float in but the small propor-
grains show little variation: coloured varieties such as tion of dense minerals will sink. These minerals are
smoky or milky quartz and amethyst occur but mostly
uncommon and study of them is only possible after
quartz is seen as clear grains.
concentrating them by dense liquid separation. They
are valuable in provenance studies (5.4.1) because
they can be characteristic of a particular source area
Feldspar
and are therefore valuable for studies of the sources of
Most igneous rocks contain feldspar as a major com- detritus. Common heavy minerals include zircon,
ponent. Feldspar is hence very common and is released tourmaline, rutile, apatite, garnet and a range of
in large quantities when granites, andesites, gabbros other metamorphic and igneous accessory minerals.
as well as some schists and gneisses break down. How-
ever, feldspar is susceptible to chemical alteration dur-
Miscellaneous minerals
ing weathering and, being softer than quartz, tends to
be abraded and broken up during transport. Feldspars Other minerals rarely occur in large quantities in
are only commonly found in circumstances where the sandstone. Most of the common minerals in igneous
chemical weathering of the bedrock has not been too silicate rocks (e.g. olivine, pyroxenes and amphiboles)
intense and the transport pathway to the site of deposi- are all too readily broken down by chemical weath-
tion is relatively short. Potassium feldspars are more ering. Oxides of iron are relatively abundant. Local
common as detrital grains than sodium- and calcium- concentrations of a particular mineral may occur
rich varieties, as they are chemically more stable when when there is a nearby source.
subjected to weathering (6.4).
2.3.2 Other components of sands
Mica and sandstones
The two commonest mica minerals, biotite and
Lithic fragments
muscovite, are relatively abundant as detrital grains
in sandstone, although muscovite is more resistant to The breakdown of pre-existing, fine- to medium-
weathering. They are derived from granitic to inter- grained igneous, metamorphic and sedimentary
mediate composition igneous rocks and from schists rocks results in sand-sized fragments. Sand-sized lithic
and gneisses where they have formed as metamorphic fragments are only found of fine to medium-grained
minerals. The platy shape of mica grains makes them rocks because by definition the mineral crystal and
distinctive in hand specimen and under the micro- grains of a coarser-grained rock type are the size of
scope. Micas tend to be concentrated in bands on sand grains or larger. Determination of the lithology