Page 227 - Handbook of Gold Exploration and Evaluation

198 Handbook of gold exploration and evaluation

Table 4.2 Wentworth scale of sediment measurement

Phi size (') Millimetres Micrometres Wentworth grade
(mm) ( m)

Cobbles
ÿ6.0 64 64 000 60.0mm
ÿ5.5 44.8 44 800
ÿ5.0 32 32 000 Coarse gravel
ÿ4.5 22.4 22 400 20.0mm
ÿ4.0 16 16 000
ÿ3.5 11.2 11 200 Medium gravel
ÿ3.0 8 8000
ÿ2.5 5.6 5600 6.0mm
ÿ2.0 4 4000
ÿ1.5 2.8 2800 Fine gravel
ÿ1.0 2 2000 2.0 mm
ÿ0.5 1.4 1400
0.0 1 1000 Coarse sand
0.5 0.71 710
1.0 0.5 500 0.6 mm
1.5 0.355 355
2.0 0.25 250 Medium sand
2.5 0.18 180 0.2 mm
3.0 0.125 125
3.5 0.090 90 Fine sand
4.0 0.063 63 0.06mm
4.5 0.045 45
5.0 0.032 32 Coarse silts
5.5 0.023 23 0.02mm
6.0 0.016 16
6.5 0.011 11.0 Medium silt
7.0 0.008 8.0
7.5 0.0055 5.5 0.006 mm
8.0 0.004 4.0 Fine silt
8.5 0.00275 2.75
9.0 0.002 2.0 0.002 mm
9.5 0.00138 1.38 Clay
10.0 0.001 1.0

inversely with the particle size. As shown in Fig. 4.2, the relationship between
micrometre and phi scale measurements is, in ascending particle size:
· Clay minerals, which comprise platy layers of alumino-silicates are about 1
nanometer (10 ÿ9 m) diameter and can be photographed only with the aid of
an electron microscope.
· Silt composition tends towards relatively inert silica in the size range 4 to 62
microns (10 ' to 4 ').
· Sands range in size from 62 to 2,000 microns (4 ' to ÿ1 ') and are almost
pure silica or quartz.