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Earth and Space Sciences · 术语表

什么是 Grain-size scale, phi scale？

又称： grain-size scale · phi scale

Definition 21.1 University Earth and Space Sciences — Year 1 · 第 21 章 — Clastic Sediments and Their Environments

The grain-size scale of sedimentology divides the diameters of grains into classes whose limits double from one class to the next, starting from $1\,\mathrm{mm}$ ; each class is named (sand, silt…). The phi scale expresses a diameter $d$ by

\varphi = -\log_2 \frac{d}{1\,\mathrm{mm}},

so that $d = 2^{-\varphi}$ mm: $1\,\mathrm{mm}$ is $\varphi = 0$ , $2\,\mathrm{mm}$ is $\varphi = -1$ , $0.25\,\mathrm{mm}$ is $\varphi = 2$ , and finer grains have larger values of $\varphi$ .

class	diameter ( $\mathrm{mm}$ )	$\varphi$
boulder	above 256	below $-8$
cobble	64 to 256	$-6$ to $-8$
pebble	4 to 64	$-2$ to $-6$
granule	2 to 4	$-1$ to $-2$
sand (very coarse, coarse, medium, fine, very fine)	$1/16$ to 2	4 to $-1$
silt (coarse, medium, fine, very fine)	$1/256$ to $1/16$	8 to 4
clay	below $1/256$ ( $3.9\,\text{µ}\mathrm{m}$ )	above 8

Table 21.1. The grain-size classes. The five classes of sand are each one phi unit wide, from very coarse (1 to

2\,\mathrm{mm}

) to very fine (

1/16

to

1/8

mm), and so are the four classes of silt. Some workers put the lower limit of silt at 9 rather than

8\,\varphi

. Gravel is the general name for grains coarser than

2\,\mathrm{mm}

, and mud for the mixture of silt and clay.

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