Geoscience Reference
In-Depth Information
41
predominant types used. A classification of sedimentary
rocks for engineering purposes is provided in
Table 7
.
S
ANDSTONE
Sandstones are formed of lithified quartz grains between
0.06 mm and 2 mm in size. A small proportion of other
mineral grains such as feldspar, mica, and rock fragments
may also be present. The grains may be closely packed
together forming a compact stone or sparsely distributed
to form a porous stone. The grains are cemented together
either by silica, calcite, iron compounds, or clay minerals.
The strength and durability of sandstone varies
considerably depending on the size of the grains, grain
packing, porosity, and nature of the cement. Sandstones
are available in a range of colours including white, grey,
brown, orange, pink, and red, which is determined by
their mineralogy and degree of weathering.
Figure
41
shows a strong and durable grey/brown
quartzitic sandstone ('York Stone') that is used for paving.
It consists of interlocking quartz grains, cemented by
silica and has relatively low porosity (6%). Figure
42
shows a softer 'Nubian Sandstone' (Cretaceous) that was
used for the construction of ancient Egyptian temples. It
consists of well sorted, fine-sized quartz grains giving a
relatively high intergrain porosity (20%). Figure
43
shows
a sandstone that has a dark grey to blue/green colour
from the inclusion of carbonaceous matter and chlorite
in the matrix between quartz grains. Figure
44
shows a
calcareous sandstone with calcite cement. This sandstone
is pink in hand specimen due the presence of red iron
41
'Northowram' quartzitic sandstone (Halifax,
England) consisting mainly of quartz grains
(grey/white/black) with traces of muscovite mica
(elongated, brightly coloured) and clay minerals
(brown); XPT, ×150.
42
42
Nubian sandstone (Egypt) consisting of quartz
grains (white) and clay minerals (brown). Macropores
are shown (yellow); PPT, ×35.
43
44
43
'Blue Pennant' sandstone (Wales) consisting of
quartz grains (grey/black) with a matrix of clay
minerals (bright) and carbonaceous matter (brown);
XPT, ×150.
44
'Cambusmore' sandstone (Scotland) consisting of
quartz grains (grey/black/white) with a veneer of
haematite (red). The stone is cemented by calcite
(pink); XPT, ×150.
