Lithic fragments, or lithics, are pieces of other
rocks
In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
that have been eroded down to
sand size and now are sand grains in a
sedimentary rock. 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 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 fragment
A rock fragment, in sedimentary geology, is a sand-sized particle or sand grain that is made up of multiple grains that are connected on the grain scale. These can include grains which are sand-sized themselves (a granitic rock fragment), or fi ...
s instead of lithic fragments.
Sandstones rich in lithic fragments are called
lithic sandstone
Lithic sandstones, or lithic arenites, or litharenites, are sandstones with a significant (>5%) component of lithic fragments, though quartz and feldspar are usually present as well, along with some clayey matrix. Lithic sandstones can have a s ...
s.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 mineral ...
), microlitic (~
andesitic), lathwork (~
basaltic), and vitric (
glassy). 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, especial ...
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, p ...
fragments, and (at times)
chert.
Metamorphic (Lm)
These can include fine-grained
schist 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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