Sandstone is a sedimentary rock composed mostly of quartz sand, but it can also contain significant amounts of feldspar, and sometimes silt and clay. When there is a significant amount of clay or silt, geologists refer to the rock as argillaceous sandstone. The color of sandstone varies, depending on its composition. Argillaceous sandstones are often gray to blue.
Because it is composed of light colored minerals, sandstone is typically light tan in color. Other elements, however, create colors in sandstone. Although feldspars are the most common minerals in igneous and metamorphic rocks, feldspars are less stable than quartz at conditions near the Earth's surface.
Feldspars in sandstones consist of the following: Plagioclase - usually showing albite twinning. Such plagioclase can be derived from both igneous and metamorphic sources. If the plagioclase also shows zoning, then it is likely from a volcanic source. Alkali Feldspar - Orthoclase and microcline are derived from both igneous and metamorphic sources. Sanidine is derived from volcanic sources. Microperthite, the intergrowth of K-rich and Na-rich alkali feldspars, is likely derived from a plutonic igneous source.
Lithic Fragments. With the exception of fragments of polycrystalline quartz, lithic fragments are generally unstable in the sedimentary environment, yet, if present in a sandstone give the best clues to provenance.
Any type of rock fragment can be found in a sandstone, but some kinds are more common due to the following factors: Areal extent in the source drainage basin.
The greater the outcrop area of the source that produces the lithic fragment, the more likely it is to occur in sediment derived from that source. Location and relief of the drainage basin. If the source is located close to the depositional basin, lithic fragments derived from the source are more likely to occur in the sediment. If the source area has high topographic relief, rates of erosion will be higher, and lithic fragments derived from the source will be more likely to occur in the sediment.
Stability of the rock fragment in the sedimentary environment. Fragments of mudrocks are relatively rare due to their mechanical weakness during transport. Similarly fragments of gabbros are rare in sandstones because the minerals they contain are chemically unstable in the sedimentary environment.
Because sandstones are usually cemented together with calcite or hematite, sandstone fragments break down easily during transport. The minerals that occur in granites, however, are more stable under conditions present near the Earth's surface, and thus granitic fragments are more common in sandstones. Volcanic rock fragments, with the exception of crystalline rhyolites, are generally unstable, but may occur if factors 1, 2 and 4 are favorable.
Size of the crystals in the fragments. In order to be present in a sandstone as a lithic fragment, the grain size of the minerals in the lithic fragment must be smaller than the grain size of the sediment.
Thus, granitic fragments will be expected to be rare, except in coarse sands, and volcanic and fine-grained metamorphic fragments will be expected to be more common. Accessory Minerals. Since it is possible that any mineral could be found in a sand or sandstone depending on the degree of mineralogical maturity, a variety of other minerals are possible.
Some of these can be useful in determining provenance of the sand. Thus the accessory minerals are usually referred to as heavy minerals. This is convenient because if the sandstone can be desegregated, then the heavy minerals can easily be separated from the quartz and feldspar on the basis of density.
The heavy minerals can be divided into three groups, as shown in the table below. Using this list, provenance of the sand can sometimes be determined to be from an igneous source or a metamorphic source.
Provenance of Accessory Minerals in Sandstones. Aegerine Augite Chromite Ilmenite Topaz. Mostly from Colorado River of Texas. Mexican Province - There are few studies of these sands, but they are expected to be similar to Rio Grande Province, reflecting a volcanic source. Glauconite occurs as green or brown sand-sized pellets in some quartz arenites, although sometimes the glauconite pellets make up a substantial portion of the rock. Glauconite has the chemical formula - K,Na,Ca 1.
The pellets are thought to originate as fecal pellets. They commonly occur in sands deposited in shallow water up to 2, m and are most common in Cambro-Ordovician and Cretaceous marine rocks, times when sea level was unusually high and the continents were flooded with epiric seas. Because glauconite contains K, the sands can sometimes be dated by the K-Ar method of radiometric dating. Tectonics and Sandstone Compositions The main factor that creates the basins necessary to form clastic sedimentary rocks is tectonics.
Once a basin is formed, the area surrounding the basin will shed its erosional debris and the sediment transported and deposited could form a sandstone. Clues to the tectonic setting in which the basin formed may be left in this accumulated sediment. Quartz Cement. Quartz cement is most common in nearly pure quartz arenites. Such rocks generally only form in environments of high energy currents, such as beach deposits, marine bars, desert dunes, and some fluvial sandbars.
Thus, it appears that most of the quartz cement is derived from the sands themselves or quartz sands in other parts of the section. Calcite Cement.
Calcite is the most common cement in sandstones, although when present, is doesn't tend to fill all pore spaces completely, but occurs as patchy cement. Calcite is soluble in surface waters, therefore calcite cemented sandstones often have their cement partially dissolved.
The sand grains in sandstone give information about the past:. Various features in sandstone are signs of the past environment:. The layers, or bedding, in sandstone are also signs of the past environment:. As a landscaping and building stone, sandstone is full of character, with warm colors. It can also be quite durable. The majority of sandstone quarried today is used as flagstones.
Unlike commercial granite , commercial sandstone is the same as what the geologists say it is. Sandstone is the official state rock of Nevada. Magnificent sandstone outcrops in the state can be seen at Valley of Fire State Park. With a great deal of heat and pressure, sandstones turn to the metamorphic rocks quartzite or gneiss, tough rocks with tightly packed mineral grains.
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