Lithic fragment (geology)




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.




Contents






  • 1 Types


    • 1.1 Igneous (Lv)


    • 1.2 Sedimentary (Ls)


    • 1.3 Metamorphic (Lm)




  • 2 References





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] By definition, intrusive igneous rock fragments can not be considered lithic fragments.



Sedimentary (Ls)


These can include shale siltstone fragments, and (at times) chert.



Metamorphic (Lm)


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





  1. ^ Dickinson, W.R., 1970, Interpreting detrital modes of graywacke and arkose: Journal of Sedimentary Petrology, v. 40, p. 695–707.


  2. ^ 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









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