Lithic fragments, or lithics, are pieces of other
rocks that have been eroded down to
sand
Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
size and now are sand grains in a
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
. They were first described and named (in their modern definitions) by
Bill Dickinson
Bill Dickinson (1917 – 7 April 1994) was a Scottish rugby union player and coach. He was appointed the first official national coach of in 1971.Bath, p133 Richard Bath points out that Dickinson's appointment made an "immediate impact" in ...
in 1970. Lithic fragments can be derived from sedimentary,
igneous
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
or
metamorphic rocks. A lithic fragment is defined using the
Gazzi-Dickinson point-counting method and being in the
sand-size fraction. Sand grains in sedimentary rocks that are fragments of larger rocks that are not identified using the Gazzi-Dickinson method are usually called
rock fragments instead of lithic fragments.
Sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicat ...
s rich in lithic fragments are called
lithic sandstones.Lithic fragments can also be define as the breakdown of preexisting ,fine -to medium grained igneous . metamorphic and sedimentary rocks results in sand -sized fragments..
Types
Igneous (Lv)
These can include granular (~
rhyolitic
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The ...
), microlitic (~
andesitic
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomin ...
), lathwork (~
basaltic
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron ( mafic lava) exposed at or very near the surface of a rocky planet or moon. More than ...
), and vitric (
glassy Glassy is an adjective meaning, of or resembling glass. Glassy may also mean:
* Glassy, amorphous metal
* ''Glassy'', a 2021 single album, or the title song by Jo Yu-ri
* Glassy phase, amorphous solid
* Glassy water, amorphous ice
* Glassy carbon, ...
). These correlations between composition and volcanic lithic fragment type are approximate, at best.
[Mathew D. Affolter, Raymond V. Ingersoll; Quantitative Analysis of Volcanic Lithic Fragments. Journal of Sedimentary Research ; 89 (6): 479–486. doi: https://doi.org/10.2110/jsr.2019.30] By definition,
intrusive igneous rock fragments can not be considered lithic fragments.
Sedimentary (Ls)
These can include
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...
fragments, and (at times)
chert
Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a ...
.
Metamorphic (Lm)
These can include fine-grained
schist
Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...
and
phyllite fragments, among others.
Image:LvMS-Lvg.jpg, Granular volcanic lithic fragment, scale in millimeters
330px, Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.
The millimetre (American and British English spelling differences#-re, -er, ...
Top picture in plane- polarized light, bottom picture in cross-polarized light.
Image:LvMS-Lvm.jpg, Microlitic volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
Image:LvMS-Lvl&Lvsm.jpg, Lathwork volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
Image:LvMS-Lvv.jpg, Vitric volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
References
Petrology
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