Download The Rock - Sandstone

The Rock - Sandstone
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Sedimentary Rock Type: Clastic
Clastic rocks are composed of fragments, or clasts, of preexisting rock.
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Related to: Shale, conglomerate, siltstone,breccias
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Color: highly variable
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Texture: Sand sized grains
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Origins: River, ocean, glacier and desert deposits
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Common Minerals: Quartz, feldspars, micas, calcite and clays
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Uses: Building material, decorative stones, tiles, tombstones,
monuments, roads, ore of silica for glass, abrasives, aquifers,
petroleum reservoirs
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Specimens:
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Sandstone is a very common sedimentary rock and perhaps the best
known sedimentary rock.
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It is formed in many environments. Just about anywhere there is
water, whether frozen or not, in a river or ocean, there is a chance to
form sandstone. Even where there is no water as in a desert, there is
sandstone formation under foot.
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As the name implies, sandstone is composed of sand.
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However there are a few other things to consider.
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What is sand?
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Sand is characterized by any grain that is 0.1 mm to 2.0 mm in size.
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Any smaller grain size and you have a shale or siltstone,
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any larger and you have either a conglomerate or a breccia.
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The grains can be composed of individual crystals of various minerals
such as quartz or feldspar or even be a sand-sized fragment of
another rock such as a granite or slate.
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A magnifying glass is usually sufficient to distinguish the general
composition of a sandstone.
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How a sandstone formed is usually important to geologists and the
minerals or rocks that are in it are critical to determine where the
source of a sandstone's composition came from.
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The roundness of the grains is also important in determining the
amount of distance the sand has been tumbled before deposition or
the closeness of the source to the final deposit.
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The lack of fine grains and mud in a sandstone indicates a relatively
high energy environment of deposition such as the wave action on a
beach, the wind sweeping across a sand dune field or the rush of a
river current.
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The grains are important to geologists and so are the minerals that
cement them together.
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Sandstone cements can influence the durability, color, porosity and
usefulness of the stone.
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Normal cementing agents include calcite, quartz (silica), clays and
gypsum.
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Silica cemented sandstone is very durable and hard.
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Calcite cemented sandstone is subject to acidic dissolution and is
more easily eroded.
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Clay and gypsum cements, which are soft minerals, tend to produce
much softer sandstone and the sand can sometimes be rubbed off in a
person's hands.
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The cementing agents also affect the porosity of a sandstone.
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When the initial sand is first deposited there are lots of open spaces
or pores.
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Water, for instance, flows right through sand due to all the pores. But
as the sand turns into rock, the cement can fill in these pores making
the sandstone less porous and less able to allow water to move
through the rock.
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Generally sandstone is a very porous rock as rocks go and will have
substantial pore spaces. Sandstone is the ideal rock for ground water
and will house substantial aquifers. Petroleum also is a fluid that
flows through sandstone and sandstone is also the best oil reservoir.
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Iron oxides, manganese oxides and other impurities can cause bright and contrasting
colors in sandstones. These colors are what gives sandstone its unique character and
ornamental desirability. The colors range from bright whites, reds, yellows, oranges and
even purples and greens. Colored sandstone is usually intricately banded in multiple
colors which enhance its aesthetic appeal.
Sandstone's banding is due to layers of sand that are deposited with differing
characteristics. Sandstone is formed in many deposits that are episodic in nature and the
resulting layers can be very different from previous layers. Sometimes the sand is courser
or finer than the previous layer and this difference causes the banding. Dunes and ripples
of sand can cut across earlier deposits and cut off those earlier bands with bands of their
new. This results in a texture call cross-bedding (see picture above) and can be very
intricate and interesting. "Picture Rock" is a type of ornamental sandstone that can
display scenic, almost picture like banding.
Sandstone deposits can be a beautiful part of the natural environment and apart of the
breath-taking views at many natural parks and preserves. The sandstone deposits of the
the Grand Canyon, Zion, Bryce, Mesa Verde, Arches, and Red Rock National Parks to
name a few in just the USA provide millions of visitors spectacular views.
Mined sandstone can be cut, polished and carved for many uses. It is used as ornamental
rocks for buildings, monuments, grave stones, bookends, beverage coasters, tiles and
many other possible uses. Sandstone can also provide silica for glass production.
Some mineral ores are found in sandstone. Uranium can concentrate in uranium minerals
in sandstone deposits and many uranium ore bodies are the result. Heavy minerals such
as rutile, gold, diamonds and others can be found in sandstones from prehistoric placer
deposits that are now worked for their economic value.
Sandstone (sometimes known as arenite) is a sedimentary rock composed mainly of
sand-sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar
because these are the most common minerals in the Earth's crust. Like sand, sandstone
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may be any color, but the most common colors are tan, brown, yellow, red, gray and
pink, white. Since sandstone beds often form highly visible cliffs and other topographic
features, certain colors of sandstone have been strongly identified with certain regions.
Some sandstones are resistant to weathering, yet are easy to work. This makes sandstone
a common building and paving material. However, some that have been used in the past,
such as the Collyhurst sandstone used in North West England, have been found less
resistant, necessitating repair and replacement in older buildings.[1] Because of the
hardness of the individual grains, uniformity of grain size and friability of their structure,
some types of sandstone are excellent materials from which to make grindstones, for
sharpening blades and other implements. Non-friable sandstone can be used to make
grindstones for grinding grain, e.g., gritstone.
Rock formations that are primarily composed of sandstone usually allow percolation of
water and other fluids and are porous enough to store large quantities, making them
valuable aquifers and petroleum reservoirs. Fine-grained aquifers, such as sandstones, are
more apt to filter out pollutants from the surface than are rocks with cracks and crevices,
such as limestone or other rocks fractured by seismic activity
Origins
Sandstones are clastic in origin (as opposed to either organic, like chalk and coal, or
chemical, like gypsum and jasper).[2] They are formed from cemented grains that may
either be fragments of a pre-existing rock or be mono-minerallic crystals. The cements
binding these grains together are typically calcite, clays and silica. Grain sizes in sands
are defined (in geology) within the range of 0.0625 mm to 2 mm (0.002-0.079 inches).
Clays and sediments with smaller grain sizes not visible with the naked eye, including
siltstones and shales, are typically called argillaceous sediments; rocks with larger grain
sizes, including breccias and conglomerates are termed rudaceous sediments.
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Red sandstone interior of Lower Antelope Canyon, Arizona, worn smooth by erosion
from flash flooding over thousands of years.
The formation of sandstone involves two principal stages. First, a layer or layers of sand
accumulates as the result of sedimentation, either from water (as in a river, lake, or sea)
or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from
suspension; i.e., ceasing to be rolled or bounced along the bottom of a body of water
(e.g., seas or rivers) or ground surface (e.g., in a desert or erg). Finally, once it has
accumulated, the sand becomes sandstone when it is compacted by pressure of overlying
deposits and cemented by the precipitation of minerals within the pore spaces between
sand grains.
The most common cementing materials are silica and calcium carbonate, which are often
derived either from dissolution or from alteration of the sand after it was buried. Colors
will usually be tan or yellow (from a blend of the clear quartz with the dark amber
feldspar content of the sand). A predominant additional colorant in the southwestern
United States is iron oxide, which imparts reddish tints ranging from pink to dark red
(terracotta), with additional manganese imparting a purplish hue. Red sandstones are also
seen in the Southwest and West of Britain, as well as central Europe and Mongolia. The
regularity of the latter favors use as a source for masonry, either as a primary building
material or as a facing stone, over other construction.
The environment where it is deposited is crucial in determining the characteristics of the
resulting sandstone, which, in finer detail, include its grain size, sorting and composition
and, in more general detail, include the rock geometry and sedimentary structures.
Principal environments of deposition may be split between terrestrial and marine, as
illustrated by the following broad groupings:
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Terrestrial environments
1.
2.
3.
4.
5.
Rivers (levees, point bars, channel sands)
Alluvial fans
Glacial outwash
Lakes
Deserts (sand dunes and ergs)
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Marine environments
1.
2.
3.
4.
5.
6.
Deltas
Beach and shoreface sands
Tidal flats
Offshore bars and sand waves
Storm deposits (tempestites)
Turbidites (submarine channels and fans)
Components
Framework grains are silicate grains that are detrital in origin, their purpose is to
support the sand.[3] Framework grains range in size from 1/16 to 2mm.[4][5] These grains
can then be classified into three different categories based on their mineral composition.
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Quartz framework grains are the dominate minerals in most sedimentary rocks;
this is because they have exceptional physical properties, such as hardness and
chemical stability.[6] These physical properties allow the quartz grains to survive
multiple recycling events, while also allowing the grains to display some degree
of rounding.[6] Quartz grains evolve from plutonic rock, which are felsic in origin
and also from older sandstones that have been recycled.
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Feldspathic framework grains are the second most abundant mineral in
sandstones.[6] Feldspar can be divided into two smaller subdivisions: alkali
feldspars and plagioclase feldspars. The different types of feldspar can be
distinguished under a petrographic microscope.[6] Below is a description of the
different types of feldspar.
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Alkali feldspar is a group of minerals in which the chemical composition
of the mineral can range from KAlSi3O8 to NaAlSi3O8, this represents a
complete solid solution.[6]
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Plagioclase feldspar is a complex group of solid solution minerals that
range in composition from NaAlSi3O8 to CaAl2Si2O8.[6]
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Photomicrograph of a volcanic sand grain; upper picture is plane-polarized light, bottom
picture is cross-polarized light, scale box at left-center is 0.25 millimeter. This type of
grain would be a main component of a lithic sandstone.
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Lithic framework grains are pieces of ancient source rock that have yet to
weather away to individual mineral grains, called lithic fragments or clasts.[6]
Lithic fragments can be any fine-grained or coarse-grained igneous, metamorphic,
or sedimentary rock.[6] Although, the most common lithic fragment found in
sedimentary rocks are clasts of volcanic rocks.[6]
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Accessory minerals are minerals that have an average abundance of less than 12% in sedimentary rocks.[6] Accessory minerals are heavier in density than
common rock-forming minerals, such as quartz and feldspar.[7] Common
accessory minerals include: micas (muscovite and biotite), olivine, pyroxene, and
corundum.[6][7]
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Heavy minerals are used to measure the amount of weathering and maturity in a
sandstone, through the ZTR index.[8] These can include Zircon, Tourmaline,
Rutile (hence 'ZTR'), Garnet, Magnetite, or other dense mineral derived from the
source rock.
[edit] Matrix
Matrix is very fine material, which is present within interstitial pore space between the
framework grains.[6] The interstitial pore space can be classified into two different
varieties. One is to call the sandstone an arenite, and the other is to call it a wacke. Below
is a definition of the differences between the two matrices.
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Arenites are texturally "clean" sandstones that are free of or have very little
matrix.[7]
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Wackes are texturally "dirty" sandstones that have a significant amount of
matrix.[5]
[edit] Cement
Cement is what binds the siliclastic framework grains together. Cement is a secondary
mineral that forms after deposition and during burial of the sandstone.[6] These cementing
materials may be either silicate minerals or non-silicate minerals, such as calcite.[6]
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Silica cement can consist of either quartz or opal minerals. Quartz is the most
common silicate mineral that acts as cement. In sandstone where there is silica
cement present the quartz grains are attached to cement, this creates a rim around
the quartz grain called overgrowth. The overgrowth retains the same
crystallographic continuity of quartz framework grain that is being cemented.
Opal cement is found in sandstones that are rich in volcanogenic materials, and
very rarely is in other sandstones.[6]
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Calcite cement is the most common carbonate cement. Calcite cement is an
assortment of smaller calcite crystals. The cement adheres itself to the framework
grains, this adhesion is what causes the framework grains to be adhered
together.[6]
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Other minerals that act as cements include: hematite, limonite, feldpsars,
anhydrite, gypsum, barite, clay minerals, and zeolite minerals.[6]
[edit] Pore Space
Pore space includes the open spaces within a rock or a soil.[9] The pore space in a rock
has a direct relationship to the porosity and permeability of the rock. The Porosity and
permeability is directly influenced by the way the sand grains are packed together.[6]
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Porosity is the percentage of bulk volume that is inhabited by interstices within a
given rock.[9] Porosity is directly influenced by the packing of even-sized
spherical grains, rearranged from loosely packed to tightest packed in
sandstones.[6]
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Permeability is the rate in which water flows, and this is measured in gallons per
day through a one square foot cross section under a unit hydraulic gradient.[9]
Hydraulic gradient is the change in depth of the water table due to the direction of
groundwater flow.
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[edit] Types of sandstone
Schematic QFL diagram showing tectonic provinces
Sandstone composed mainly of quartz grains
All sandstone are composed of the same general minerals. These minerals make up the
framework components of the sandstones. Such components are quartz, feldspars, and
lithic fragments. Matrix may also be present in the interstitial spaces between the
framework grains.[6] Below is a list of several major groups of sandstones. These groups
are divided based on mineralogy and texture. Even though sandstones have very simple
compositions which are based on framework grains, geologists have not been able to
agree on a specific, right way, to classify sandstones.[6] Sandstone classifications are
typically done by point-counting a thin section using a method like the Gazzi-Dickinson
Method. The composition of a sandstone can have important information regarding the
genesis of the sediment when use with a triangle Quartz, Feldspar, Lithic Fragment (QFL
diagrams). Many geologist however do not agree on how to separate the triangle parts
into the single components so that the framework grains can be plotted.[6]Therefore, there
have been many published ways to classify sandstones, all of which are similar in their
general format.
Visual aids are diagrams that allow geologists to interpret different characteristics about
a sandstone. The following QFL chart and the sandstone provenance model correspond
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with each other therefore, when the QFL chart is plotted those points can the be plotted
on the sandstone provenance model. The stage of textural maturity chart illustrates the
different stages that a sandstone goes through.
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A QFL chart is a representation of the framework grains and matrix that is present
in a sandstone. This chart is similar to those used in igneous petrology. When
plotted correctly, this model of analysis creates for a meaningful quantitative
classification of sandstones.[3]
A sandstone provenance chart allows geologists to visually interpret the different
types of places sandstones can originate from.
A stage of textural maturity is a chart that shows the different stages of
sandstones. This chart shows the difference between immature, submature,
mature, and supermature sandstones. As the sandstone becomes more mature
grains become more rounded, and there is less clay that makes up the matrix of
the rock.[6]
[edit] Dott (1964) Classification Scheme
Dott's (1964) sandstone classification scheme is one of many classification scheme used
by geologists for classifying sandstones. Dott's scheme is a modification of Gilbert's
classification of silicate sandstones, and it incorporates R.L. Folk's dual textural and
compositional maturity concepts into one classification system.[10] The philosophy behind
combining Gilbert's classification scheme and R.L. Folk's classification scheme is that it
is better able to "portray the continuous nature of textural variation from mudstone to
arenite and from stable to unstable grain composition".[10] Dott's classification scheme is
based on the mineralogy of framework grains, and on the type of matrix present in
between the framework grains.
In this specific classification scheme, Dott has set the boundary between arenite and
wackes at 15% matrix. In addition to setting a boundary for what the matrix is, Dott also
breaks up the different types framework grains that can be present in a sandstone into
three major categories: quartz, feldspar, and lithic grains.[6]
Arenites describe sandstone that have less than 15% clay matrix in between the
framework grains.
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Quartz Arenite are sandstones that contain more than 90% of siliceous grains.
Grains can include quartz or chert rock fragments.[6] Quartz arenites are texturally
mature to supermature sandstones. These pure quartz sands result from extensive
weathering that occurred before and during transport. This weathering removed
everything but quartz grains, the most stable mineral.They are commonly
affiliated with rocks that are deposited in a stable cratonic environment, such as
eolian beaches or shelf environments.[6] Quartz arenites emanate from multiple
recycling of quarts grains, generally as sedimentary source rocks and less
regularly as first-cycle deposits derived form primary crystalline or metamorphic
rocks.[6]
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
Feldspathic Arenites are sandstones that contain less than 90% quartz, and more
feldspar than unstable lithic fragments, and minor accessory minerals.[6]
Feldspathic sandstones are commonly immature or sub-mature.[6] These
sandstones occur in association with cratonic or stable shelf settings.[6]
Feldspathic sandstones are derived from granitic-type, primary crystalline,
rocks.[6] If the sandstone is dominantly plagioclase, then it is igneous in origin.[6]

Lithic Arenites are characterized by generally high content of unstable lithic
fragments. Examples include volcanic and metamorphic clasts, though stable
clasts such as chert are common in lithic arenites.[6] This type of rock contains
less than 90% quartz grains and more unstable rock fragments than feldspars.[6]
They are commonly immature to submature texturally.[6] They are associated with
fluvial conglomerates and other fluvial deposits, or in deeper water marine
conglomerates.[6] They are emanate under conditions that produce large volumes
of unstable material, derived from fine-grained rocks, mostly shales, volcanic
rocks, and metamorphic rock.[6]
Wacke describes sandstones that contain more than 15% clay matrix in between
framework grains.

Quartz Wacke are uncommon because quartz arenites are texturally mature to
supermature.[6]
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Felspathic Wacke are feldspathic sandstone that contain a matrix that is greater
than 15%.[6]
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Lithic Wacke is a sandstone that has a matrix greater than 15%.[6]
Arkose sandstones are more than 25 percent feldspar.[2] The grains tend to be poorly
rounded and less well sorted than those of pure quartz sandstones. These feldspar-rich
sandstones come from rapidly eroding granitic and metamorphic terrains where chemical
weathering is subordinate to physical weathering.
Graywacke sandstones are a heterogeneous mixture of lithic fragments and angular
grains of quartz and feldspar, and/or grains surrounded by a fine-grained clay matrix.
Much of this matrix is formed by relatively soft fragments, such as shale and some
volcanic rocks, that are chemically altered and physically compacted after deep burial of
the sandstone formation.
Eolianite is a term used for a rock which is composed of sand grains that show signs of
significant transportation by wind. These have usually been deposited in desert
environments. They are commonly extremely well sorted and rich in quartz.
Oolite is more a limestone than a sandstone, but is made of sand-sized carbonate ooids,
and is common in saline beaches with gentle wave action.
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