Sedimentary Rocks
On Quartz and sedimentary rocks Silicon and oxygen, the two most common
chemical
elements in the Earth's crust, combine as silicon dioxide to form
the mineral
quartz. SEDIMENTARY ROCKS, are Rocks that are formed by the
compaction of
sediments or by the crystallization of the dissolved minerals.
Quartz is the
most abundant mineral in the Earth's crust. Quartz has been
found in meteorites
and in some rocks collected on the moon. Quartz has the
chemical formula SiO2.
There are many varieties of quartz, which occurs
in nearly all types of igneous,
metamorphic and sedimentary rocks. It is an
essential mineral in granites,
granodiorites and rhyolites. Metamorphism of
quartz-bearing igneous and
sedimentary rocks typically increases the amount
of quartz and its grain size.
Quartz is very resistant to weathering and,
therefore, concentrates as
sandstones and other detrital rocks. Most sands
are weathered fragments of
quartz. Sandstone and quartzite are the same
materials that are re-lithified
(compressed into rock). Quartz has many
commercial applications. In science and
industry, quartz sand is used for
manufacturing sandpaper and other abrasives,
polishing powders and soaps. It
is used to make building materials,
heat-resistant materials known as
refractories and for the bearings of precision
instruments. Quartz sand is
also used in the manufacture of porcelain, glass and
metal casting molds.
Precision-cut plates of quartz are used for frequency
control in radios, TVs,
clocks and watches. Compositionally, quartz is usually
quite pure, with only
traces of other elements like aluminum, sodium, potassium
and lithium. Quartz
is found as large crystals that are often beautifully
colored by impurities.
The many varieties of quartz are due to formation of
different geometric
arrangements of its tetrahedral crystals. This accounts for
different crystal
structures, and, therefore, different physical properties. To
form Quartz
sandstone, rock particles & remains of living things are
deposited in the
sea by rivers and waves. These sediments accumulate layer by
layer, over
thousands or millions of years, forming thick deposits on land or on
the sea
floor. The weight of the overlying sediments compact those below. Over
time,
they form sedimentary rocks. Sedimentary rocks usually occur in shallow
parts
of the sea or in lakes in desert areas where evaporation is higher
than
precipitation. As evaporation takes place, water is lost and the
dissolved
minerals form crystals. As evaporation continues, more crystals
form and
accumulate on the sea or lake floor, becoming sedimentary rocks,
i.e. coal. They
have a layered appearance because the materials that form
them are deposited in
layers. Plant and animal fossils can be found in many
sedimentary rocks. Color:
Very variable; frequently red, brown, greenish,
yellow, gray, white. Texture:
Medium-grained. Usually well sorted, that
is grains all about the same size;
grains sub angular to rounded (sandstone).
Structure: Bedding usually apparent;
current bedding and ripple marks common;
graded bedding may occur. Concretions
and fossils may be found. Mineralogy:
Quartz is the main component but is often
accompanied by feldspar, mica or
other minerals. The grains may be cemented by
silica, calcite or iron oxides.
Field relations: Sandstones are associated with
most other sedimentary rocks.
Most sands accumulated either in water, usually
the sea, or as wind-blown
deposits in arid continental areas. Desert sandstones
tend to be red, and the
individual sand grains are often almost spherical and
polished."