SANDSTONE
Sandstone is the 2nd-most common siliciclastic sedimentary rock on Earth. They have both environmental and economic
importance in human society.
Sandstones are composed of sand-sized grains
(between 2 mm and 0.0625 mm in size). The vast majority of sandstones are
quartzose sandstones (dominated by quartz sand), but feldspathic
sandstones (dominated by K-feldspar sand) and lithic sandstones (dominated by
sand grains composed of rock fragments) are not uncommon.
Sandstones vary in color & coarseness.
Common sandstone colors are brownish, grayish, reddish, orangish, and
whitish. Coarse-grained sandstones will feel the roughest when rubbed
with a finger. Fine-grained sandstones are far less gritty feeling.
Some sandstones are hard, dense, and tight. This is the result of mineral cement (usually
quartz) completely filling up the spaces between sand grains. Many sandstones have not had cement
completely filling the empty spaces between sand grains. So, the degree
of porosity in sandstones varies.
Moderately- to poorly-cemented sandstones often have their porosity
filled with water, oil, or natural gas.
Many sources of groundwater (aquifers) in the world
are sand or sandstone deposits. Many economic petroleum fields have
sandstones as the reservoir rocks for the oil & natural gas.
Well-cemented sandstones have traditionally been used as building stones in
various parts of the world.
Sandstone (quartzose sandstone)
Graywacke (2.5 cm
across) - this sandstone is a graywacke,
which generally means a "dirty sandstone". This refers to the rock having a significant,
fine-grained, muddy component mixed in with the sand grains. “Graywacke”
is probably the most misused & misunderstood term in all of geology.
The word often appears in older geologic literature, and modern geologists have
been encouraged to avoid using the term.