Page 32 - Sedimentology and Stratigraphy

Name:
Compositor
Compositor
Name:
page
ARaju
ARaju
8:14pm
26.2.2009
19
19
26.2.2009
8:14pm
page
26.2.2009 8:14pm Compositor Name: ARaju
Proof
and
and
Nichols/Sedimentology
Stratigraphy
9781405193795_4_0
9781405193795_4_0
Stratigraphy
Nichols/Sedimentology
Final Proof page 19
Final
Proof
Final
Nichols/Sedimentology and Stratigraphy 9781405193795_4_002
02
02
Sand and Sandstone 19
result of alteration of some of the iron to oxides and ment. The bedrock must itself be composed of crystals
hydroxides. or particles that are smaller than sand-size: granite
consists of crystals that are sand-sized or larger, and
so cannot occur as lithic clasts in sands, but its fine-
Oxides and sulphides
grained equivalent, rhyolite, can occur as grains.
The vast majority of natural oxide and sulphide miner- Lithic fragments of fine-grained metamorphic and
als are opaque, and simply appear as black grains sedimentary rocks can also be common.
under plane-polarised light. The iron oxide haematite
is particularly common, occurring as particles that Chert and chalcedony
range down to a fine dust around the edges of grains
and scattered in the matrix. The edges of haematite Under plane-polarised light, chert (3.3) looks very
grains will often look brownish-red. Magnetite, also an much like quartz, because it is also composed of silica.
iron oxide, occurs as a minor component of many The difference is that the silica in chert is in an amor-
igneous rocks and is quite distinctive because it occurs phous or microcrystalline form: under cross-polars it
as euhedral, bipyramidal crystals, which appear as therefore often appears to be highly speckled black,
four or eight-sided, equant black grains in thin-section. white and grey, with individual ‘crystals’ too small to
Iron hydroxides, limonite and goethite, which are yel- be resolved under a normal petrographic microscope.
lowish brown in hand specimen, appear to have brown Chalcedony is also a form of silica that can readily be
edges in thin-section. identified in thin-section because it has a radial struc-
Pyrite is an iron sulphide that may crystallise ture when viewed under cross-polars; fine black and
within sediments. Although a metallic gold colour as white lines radiate from the centre, becoming lighter
a fully-formed crystal, fine particles of pyrite appear and darker as the grain is rotated.
black, and in thin-section this mineral often appears
as black specks, with the larger crystals showing the
Organic material
cubic crystal shape of the mineral. Locally, other
sulphides and oxides can be present, for example the Carbonaceous material, the remains of plants, is
tin ore, cassiterite, which occurs as a placer mineral brown in colour, varying from black and opaque to
(minerals that concentrate at the bottom of a flow due translucent reddish brown in thin-section. The paler
to their higher density). grains can resemble a mineral, but are always black
under cross-polars. The shape and size is extremely
variable and some material may appear fibrous. Coal
Heavy minerals
is a sedimentary rock made up largely of organic
A thin-section of a sandstone is unlikely to contain material: the thin-section study of coal is a specialised
many heavy mineral grains. Zircon is the most fre- subject that can yield information about the vegeta-
quently encountered member of this group: it is an tion that it formed from and its burial history.
extremely resistant mineral that can survive weath-
ering and long distances of transport. Grains are Sedimentary rock fragments
equant to elongate, colourless and easily recognised
by their very high relief: the edges of a zircon grain Clasts of claystone, siltstone or limestone may be pres-
will appear as thick, black lines. Other relatively com- ent in a sandstone, and a first stage of recognition of
mon heavy minerals are rutile, apatite, tourmaline them is that they commonly appear rather ‘dirty’
and sphene. under plane-polarised light. Very fine particles of
clay and iron oxide in a lithic fragment will make it
appear brownish in thin-section, and if the grain is
2.3.7 Lithic grains made entirely of clay it may be dark brown. Siltstone
is most commonly composed of quartz grains, which
Not every grain in a sandstone is an individual will be evident as black and white spots under crossed
mineral: the breakdown of bedrock by weathering polars: individual silt grains may be identified if a
leads to the formation of sand-sized fragments of the high-power magnification is used to reveal the edges
original rock that can be incorporated into a sedi- of the silt-sized clasts.