Clastic rock
Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material (matrix) composition, and texture.
An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream.
They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition of mudrocks.
The majority of silica cements are composed of quartz, but can include chert, opal, feldspars and zeolites.
[3] Composition includes the chemical and mineralogic make-up of the single or varied fragments and the cementing material (matrix) holding the clasts together as a rock.
While grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories: conglomerates, sandstones, and mudrocks.
Conglomerates are common in stratigraphic successions of most, if not all, ages but only make up one percent or less, by weight, of the total sedimentary rock mass.
[3] Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together.
These sand-size particles are often quartz but there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition.
Classification schemes vary widely, but most geologists have adopted the Dott scheme,[5][better source needed] which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains.
Classification schemes for mudrocks tend to vary, but most are based on the grain size of the major constituents.
[6] According to Blatt, Middleton and Murray [7] mudrocks that are composed mainly of silt particles are classified as siltstones.
The plate-like shape of clay allows its particles to stack up one on top of another, creating laminae or beds.
Siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds.
The process of turning loose sediment into hard sedimentary rocks is called lithification.
During the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock.
[3] Due to the shallow depths, sediments undergo only minor compaction and grain rearrangement during this stage.
Despite being close to the surface, eogenesis does provide conditions for important mineralogical changes to occur.
Other important reactions include the formation of chlorite, glauconite, illite and iron oxide (if oxygenated pore water is present).
The precipitation of potassium feldspar, quartz overgrowths, and carbonate cements also occurs under marine conditions.
As sediments are buried deeper, load pressures become greater resulting in tight grain packing and bed thinning.
In this process minerals crystallize from watery solutions that percolate through the pores between grain of sediment.
Dissolution of framework silicate grains and previously formed carbonate cement may occur during deep burial.
Rock fragments and silicate minerals of low stability, such as plagioclase feldspar, pyroxenes, and amphiboles, may dissolve as a result of increasing burial temperatures and the presence of organic acids in pore waters.
In the process of burial, it is possible that siliciclastic deposits may subsequently be uplifted as a result of a mountain building event or erosion.
Because the process brings material to or closer to the surface, sediments that undergo uplift are subjected to lower temperatures and pressures as well as slightly acidic rain water.
Occasionally, metamorphic rocks can be brecciated via hydrothermal fluids, forming a hydrofracture breccia.
This is particularly prominent in epithermal ore deposits and is associated with alteration zones around many intrusive rocks, especially granites.
