Lithic fragments, or lithics, are pieces of other
rocks 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 in 1970.[1] 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 fragments instead of lithic fragments.
Sandstones rich in lithic fragments are called
lithic sandstones.
Types
Igneous (Lv)
These can include granular (~
rhyolitic), microlitic (~
andesitic), lathwork (~
basaltic), and vitric (
glassy). These correlations between composition and volcanic lithic fragment type are approximate, at best.[2][3] By definition,
intrusive igneous rock fragments can not be considered lithic fragments.
These can include fine-grained
schist and
phyllite fragments, among others.
Granular volcanic lithic fragment, scale in
millimeters Top picture in plane-
polarized light, bottom picture in cross-polarized light.
Microlitic volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
Lathwork volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
Vitric volcanic lithic fragment, scale in millimeters. Top picture in plane-polarized light, bottom picture in cross-polarized light.
References
^Dickinson, W. R. (1970). "Interpreting detrital modes of graywacke and arkose". Journal of Sedimentary Petrology. 40: 695–707.
^Affolter, M.D. and Hendrix, M. S., 2004, Correlations between volcanic lithic fragments and volcanic rock, Geological Society of America Abstracts with Programs, Vol. 36, No. 5, p. 370
^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