Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
The formal definition of particle sizes used for describing sediment wasdefined by Udden (1914) and Wentworth (1922, 1925).
Sediment size is commonly described quantitatively using the Krumbein (1934,1938) phi scale, which has a couple of odd quirks:
Despite proposals to change it to a decimal system, the Krumbein scalesurvives. The scale is defined as size (mm) = 2-phi. A one-mm grainhas a phi of zero, phi = 1 means 1/2 mm, phi = 5 = 1/25 = 1/32 mm,and so on. Grains larger than a millimeter have negative phi, so a grain withphi = -3 has a diameter of 8 mm.
| Size (phi) | Diameter | Particle Name | Aggregate Name | Rock Name |
| -12 - -8 | 4.1 m - 26 cm | Boulder | Gravel | Conglomerate |
| -8 - -6 | 26 - 6.4 cm | Cobble | ||
| -6 - -2 | 6.4 cm - 4 mm | Pebble | ||
| -2 - -1 | 4 - 2 mm | Granule | ||
| -1 - 4 | 2 - 1/16 (.06) mm | Sand | Sand | Sandstone |
| 4 - 8 | 1/16 (.06) - 1/256 (.004) mm | Silt | Mud | Mudrocks |
| 8 and up | less than 1/256 (.004) mm | Clay |
Granule-sized aggregate and the resulting coarse sandstone or fineconglomerate are sometimes termed grit.
Within each particle class, phi classes are described as very fine, fine,medium, coarse, and very coarse, with the number of terms adapted to the numberof classes. There is no very coarse silt or very fine boulders. The notion ofcalling a rock between 26 and 51 cm in diameter a "fine boulder" seemsa bit odd, but that's the term.
With the recognition of impact breccias and large landslides, the need hasarisen to describe chunks bigger than boulders. Blair and McPherson (1999)extended the size scale upward as follows. Aggregates are termed megagraveland the resulting rocks are megaconglomerates, although megabrecciais probably more apt, since these pieces are not likely to be roundedappreciably by transport.
| Size (phi) | Diameter | Particle Name |
| -30 - -25 | 1076 - 33.6 km | Megalith |
| -25 - -20 | 33.6 - 1 km | Monolth |
| -20 - -16 | 1 km - 66 m | Slab |
| -16 - -12 | 66 - 4.1 m | Block |
Within each particle class, again, phi classes are described as very fine,fine, medium, coarse, and very coarse, with the number of terms adapted to thenumber of classes. The notion of calling a rock between 34 and 67 km in diametera "very fine megalith" seems truly weird. I want to see the sievescreen they use to separate that size fraction.
"Block" and "megalith" seem reasonable, but"slab" and "monolith" seem to convey unintended shapeconnotations. Maybe "block," "megalith," "gigalith,"and "teralith" would have been a better set of terms.
Terms from Greek and Latin have been used to describe mud, sand and gravel:
| Size | Greek | Latin |
| Gravel - Conglomerate | Psephite | Rudite |
| Sand - Sandstone | Psammite | Arenite |
| Mud - Mudrock | Pelite | Lutite |
Psephite is probably the most rarely used term. Psammite and pelite arewidely used in metamorphic terminology where the original rock type can only beestimated from its chemistry. The Latin terms are widely used in dealing withclastic carbonate rocks; a calcirudite is a coarse carbonate breccia, forexample reef talus, and a calcilutite is calcareous mud.
Sedimentary rocks made of silt- and clay-sized particles, collectively calledmudrocks, and are the most abundant sedimentary rocks, making up twothirds of the total. Surprisingly little is known about their formation becausethe grain sizes are so small.
Maturity signifies removal of unstable ingredients, long weathering and longmechanical reworking.
Stability roughly parallels Bowen's series for igneous minerals, but otherminerals also occur. In rough increasing order of stability we have:
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Created 26 October 2001, Last Update