Editing Silt (section)

==Description==
Silt is [[Detritus (geology)|detritus]] (fragments of [[Weathering|weathered]] and [[eroded]] rock) with properties intermediate between [[sand]] and [[clay]]. A more precise definition of silt used by geologists is that it is detrital particles with sizes between 1/256 and 1/16&nbsp;mm (about 4 to 63 microns).<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |page=381}}</ref> This corresponds to particles between 8 and 4 phi units on the [[Krumbein phi scale]].<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=silt [sed]}}</ref><ref>{{cite book |last1=Potter |first1=Paul Edwin |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |title=Sedimentology of shale : study guide and reference source |date=1980 |publisher=Springer-Verlag |location=New York |isbn=0387904301 |page=15}}</ref> Other geologists define silt as detrital particles between 2 and 63 microns or 9 to 4 phi units.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} A third definition is that silt is fine-grained detrital material composed of [[quartz]] rather than [[clay minerals]].{{sfn|Potter|Maynard|Pryor|1980|p=13}} Since most clay mineral particles are smaller than 2 microns,{{sfn|Potter|Maynard|Pryor|1980|p=15}} while most detrital particles between 2 and 63 microns in size are composed of broken quartz grains, there is good agreement between these definitions in practice.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

The upper size limit of 1/16&nbsp;mm or 63 microns corresponds to the smallest particles that can be discerned with the unaided eye.{{sfn|Jackson|1997|loc="silt [sed]"}} It also corresponds to a ''Tanner gap'' in the distribution of particle sizes in [[sediments]]: Particles between 120 and 30 microns in size are scarce in most sediments, suggesting that the distinction between sand and silt has physical significance.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}} As noted above, the lower limit of 2 to 4 microns corresponds to the transition from particles that are predominantly broken quartz grains to particles that are predominantly clay mineral particles.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Assallay and coinvestigators further divide silt into three size ranges: C (2–5 microns), which represents post-glacial clays and desert dust; D1 (20–30 microns) representing "traditional" [[loess]]; and D2 (60 microns) representing the very coarse North African loess.{{sfn|Assallay|Rogers|Smalley|Jefferson|1998}}

Silt can be distinguished from clay in the [[Field work|field]] by its lack of plasticity or cohesiveness and by its grain size. Silt grains are large enough to give silt a gritty feel,{{sfn|Potter|Maynard|Pryor|1980|p=15}} particularly if a sample is placed between the teeth. Clay-size particles feel smooth between the teeth.<ref>{{cite book |last1=Tucker |first1=Maurice E. |title=Sedimentary rocks in the field : a practical guide |date=2011 |publisher=Wiley-Blackwell |location=Chichester, West Sussex |isbn=9780470689165 |page=160 |edition=4th}}</ref> The proportions of coarse and fine silt in a sediment sample are determined more precisely in the laboratory using the pipette method, which is based on settling rate via [[Stokes' law]] and gives the particle size distribution accordingly.{{sfn|Blatt|Middleton|Murray|1980|p=63}} The mineral composition of silt particles can be determined with a [[petrographic microscope]] for grain sizes as low as 10 microns.{{sfn|Blatt|Middleton|Murray|1980|p=305}}

''Vadose silt'' is silt-sized [[calcite]] crystals found in pore spaces and vugs in [[limestone]]. This is emplaced as sediment is carried through the [[vadose zone]] to be deposited in pore space.{{sfn|Blatt|Middleton|Murray|1980|pp=492-493}}

=== Definitions ===
'''ASTM''' American Standard of Testing Materials: 200 sieve – 0.005&nbsp;mm.

'''USDA''' United States Department of Agriculture 0.05–0.002&nbsp;mm.

'''ISSS''' International Society of Soil Science  0.02–0.002&nbsp;mm.

Civil engineers in the United States define silt as material made of particles that pass a number 200 sieve (0.074&nbsp;mm or less) but show little plasticity when wet and little cohesion when air-dried.{{sfn|Jackson|1997|loc="silt [eng]"}} The International Society of Soil Science (ISSS) defines silt as soil containing 80% or more of particles between 0.002&nbsp;mm to 0.02&nbsp;mm in size{{sfn|Jackson|1997|loc="silt [soil]"}} while the U.S. Department of Agriculture puts the cutoff at 0.05mm.<ref>{{cite web |url=http://soils.usda.gov/technical/handbook/contents/part618.html#43 |title=Particle Size (618.43) |work=National Soil Survey Handbook Part 618 (42-55) Soil Properties and Qualities |publisher= - Natural Resource Conservation Service |access-date=2006-05-31 |url-status=dead |archive-url=https://web.archive.org/web/20060527111746/http://soils.usda.gov/technical/handbook/contents/part618.html#43 |archive-date=2006-05-27 }}</ref> The term ''silt'' is also used informally for material containing much sand and clay as well as silt-sized particles, or for mud suspended in water.{{sfn|Jackson|1997|loc="silt [sed]"}}