About Sandstone
Sandstone is a sedimentary rock composed of small grains cemented by siliceous, felspathic, or calcareous cementing material. The durability of rock depends on the cementing material. Sandstone is often formed in layers and has varied applications as building stones. This coarse-grained sedimentary rock is formed by the consolidation and aggregation of sand and held together by a natural cement, such as silica.
It is an extremely hard and tough material and consists of consolidated masses of sand deposited by moving water or by wind. Some of the sandstone are so homogeneous and soft that they are capable of receiving most elaborate carving and filigree work. The color of the rock is largely determined by the cementing material - iron oxides produces red or reddish-brown sandstone, and the other materials produce sandstone in white, grayish or yellowish sandstone.
Peculiar about Sandstone
Natural Sandstone is believed to be the youngest of the quartz-based stones, such that each stone has a different level of porosity, hardness and compressive strength. Its texture is noteworthy and the medium sized grains are all of the same size.
The color of sandstone is variable and depends upon the composition of the binding material, and therefore varies from red, brown, greenish, yellow, gray and white.
Sandstone textures and compositions may be used to interpret many things about the history of the sand, including source area lithology, paleoclimate, tectonic activity, processes acting in the depositional basin, and time duration in the basin. Remember that the source area is the land which is weathering and eroding to supply terrigenous debris to the depositional basin.
Sandstone Chemical Properties
The chemical constitution of sandstone is the same as that of sand, the rock is thus composed essentially of quartz. The natural cementing material that binds the sand together as rock is usually composed of silica, calcium carbonate, or iron oxide. Chemically sandstone is very resistant Mono-Mineralic rock, with silica as the principal. The percentage of each constituents is as follows:
SiO2 93-94%
Iron (Fe2O3) 1.5%-1.6%
Alumina (Al2O3) 1.4 to 1.5%
Soda (Na2O) and Potash (Kro) 1.0% to 1.2%
Lime (CaO) 0.8% to 0.9%
Magnesia (MgO) 0.2 to 0.25%
Loss On Ignition (LOI) 1.0% to 1.2%
They are highly resistant to acids, alkalis and thermal impact.
Sandstone Physical Properties
Color-The color varies from red, green, yellow, gray and white. The variation is result of the binding material and its percentage constituent.
Water Absorption-The capacity of water absorption is not more than 1.0%
Hardness-Lies between 6 to 7 on Moh's Scale Density 2.32 to 2.42 Kg/m3
Porosity-The porosity varies from low to very low.
Compressive Strength-Varies from 365 to 460 Kg/m2
Sandstone Composition
Sandstones are classified on the basis of the composition of their grains. Three components are considered:
Quartz grains
Feldspar grains
Rock fragment grains
Based on the percentage composition of these components, the color and the physical properties of the various varieties of sand stone varies.
Major types of Sandstone
Based on the variation in composition and cementing material the sandstone has been categorized unto three major types, which are as follows:
Quartz sandstone (also called quartz arenite)
Arkose
Litharenite or lithic sandstone (commonly but imprecisely called graywacke)
Each type of sandstone has some implication about its depositional history - Quartz sandstone implies a long time in the depositional basin; Arkose implies a short time in the depositional basin (because feldspar typically weathers quickly to clay). Also implies rapid erosion, arid climate, tectonic activity, steep slopes; Litharenite implies rapid erosion, temperate or arid (not humid) climate.
Quartz sandstone or quartz arenite is dominated by quartz grains; arkose is dominated by feldspar grains (usually potassium feldspar); and graywacke is dominated by rock fragment grains.
Quartz Sandstone
Quartz sandstone formed as a result of the extreme weathering and sorting of a sediment until everything that can be removed has been removed. The high content of quartz is a result of removal of feldspar and lithic. This is done by the complete chemical weathering and then the final removal of the clay takes placed in high energy environments. Tidal sand bars that accumulate large bodies of quartz sand is yet another situation that leads to formation of sandstone. Sand-sized quartz grains could come from the weathering of source area rocks such as granite, gneiss, or other sandstones which contain quartz.
Arkose Sandstone
Arkose, a sandstone derived from disintegration of granite or gneiss, and characterized by high feldspar content. This is thus the quartz sandstone containing over a quarter feldspar with iron oxide cement. Micas may also be present. Bedding is sometimes present, but fossils are rare. It effervesces slightly in dilute hydrochloric acid thus indicating calcite cement. Its color is usually in the shades of buff, brownish-gray or pink. Arkose sandstone finds its applications in building stone, and millstones for grinding corn.
Litharenite or Lithic Sandstone
Litharenite or lithic sandstone is commonly known by the name of graywacke. It is predominantly composed of dark sand-sized rock fragments, with some mica, quartz, and feldspar grains in a clay-rich matrix. A litharenite is composed of sand-sized rock fragments.
Sandstone Textures
Sandstones have different surface textures, which varies according to their formation. However now a days the stones are fabricated as the desired texture and the usage. There are six main types of surfaces that are in use.
Natural-The natural surface texture is achieved by ripping along its line of cleavage to reveal the natural grain and texture. This texture gives an undulating surface with great character. The surface comes with natural clefts giving a very natural look.
Flamed-texture provides a rough surface. The roughness in the surface is a result of bursting of crystals when the stone is heated. Such a surface gives an irregular textured finish.
Polished-Polished surface texture is a reflection of polished crystals. Such texture brings out the brilliant colors and grains of natural stones. The shine on stone surface comes from polishing bricks and powders used during fabrication and not from any coating.
Honed-The honed texture is produced by grinding a surface with high grit material to a uniform specification, such that it does not produce a reflective surface. Thus honed stone colors are not as vibrant as polished stones. A smooth finish with a slight sheen is produced by using a polishing head. This surface is very smooth, but often very porous.
Bush Hammered-A pounding action that develops a textured surface. The top surface is pneumatically tooled to produce a pitted or grooved surface finish.
Sawn-Sawn surface is coarsely polished leaving a semi-smooth, regular finish. It is done by using a gang saw.
Sand Blasted-Producing a finish similar to cleft, sand blasting involves projecting a high-pressure airline coarse-grained grit, onto the top surface of the stone. It is characterized by a textured surface with a matte gloss.
