NationMaster - Encyclopedia: Grain size

Grain size refers to the physical dimensions of particles of rock or other solid. This is different from the crystallite size, which is the size of a single crystal inside the solid (a grain can be made of several single crystals).

Grain sizes can range from very small colloidal particles, through clay, silt, sand, and gravel, to boulders.

Size ranges define limits of classes that are given names in the Wentworth scale used in the United States. The Krumbein phi (φ) scale, created by W. C. Krumbein is logarithmic and computed by the equation:

φ = - log 2 (grain size in mm)

φ scale	Size range (metric)	Size range (approx. inches)	Aggregate name (Wentworth Class)	Other names
<−8	> 256 mm	> 10 in	Boulder
−6 to −8	64–256 mm	2.5–10 in	Cobble
−5 to −6	32–64 mm	1.2–2.5 in	Very coarse gravel	Pebble
−4 to −5	16–32 mm	0.6–1.2 in	Coarse gravel	Pebble
−3 to −4	8–16 mm	0.3–0.6 in	Medium gravel	Pebble
−2 to −3	4–8 mm	0.15–0.3 in	Fine gravel	Pebble
−1 to −2	2–4 mm	0.08–0.15 in	Very fine gravel	Granule
0 to −1	1–2 mm	0.04–0.08 in	Very coarse sand
1 to 0	0.5–1 mm	0.02–0.04 in	Coarse sand
2 to 1	0.25–0.5 mm	0.01–0.02 in	Medium sand
3 to 2	0.125–0.25 mm	0.005–0.01 in	Fine sand
4 to 3	0.063–0.125 mm	0.002–0.005 in	Very fine sand
8 to 4	0.004–0.063 mm	0.00015–0.002 in	Silt
> 8	0.001–0.004 mm	0.0004–0.00015 in	Clay
	< 0.001 mm	< 0.0004 in	Colloid

In some schemes "gravel" is anything larger than sand (>2.0 mm), and includes "granule", "pebble", "cobble", and "boulder" in the above table. In this scheme, "pebble" covers the size range 4 to 64 mm (−2 to −6 φ).

Categories: Sedimentary rocks

Results from FactBites:

Metallurgical Testing, Metallography Examination, Image Analysis, Leco, Metallography, India (938 words)
	A material's grain size is important as it affects its mechanical properties.
	In most materials, a refined grain structure gives enhanced toughness properties and alloying elements are deliberately added during the steel-making process to assist in grain refinement.
	Grain size is determined from a polished and etched sample using optical microscopy at a magnification of 100X.

NationMaster - Encyclopedia: Grain size (0 words)
	The Krumbein and Monk equation is used to estimate the permeability (in darcies) of a sediment from a grain size analysis.
	This is true because grain size is a powerful tool for describing a site’s geomorphic setting, interpreting the geomorphic significance of fluid dynamics in the natural environment, and distinguishing local versus regional sediment transport mechanisms as well as because grain size is a dominant controlling factor in sediment geochemistry.
	The grain size distribution of a sediment sample is determined by any one of a number of techniques or combination of techniques depending on the range of sizes present in a sample.

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