Laboratory Manual for Introductory Geology 4e

TABLE 4.1 Common grain size terminology
Grain size term Practical definition Approximate size range
Coarse grained Grains are large enough so you can identify the minerals or grains present. Larger than 5 mm
Medium grained Individual grains can be seen with the naked eye but are too small to be identified. 0.1–4.9 mm
Fine grained
Very fine grained
Individual grains are too small to be seen with the naked eye or identified with a
hand lens; rock feels “gritty” to the edge of a fingernail.
Individual grains cannot be seen with the naked eye or a hand lens; rock feels
smooth to the edge of a fingernail.
Smaller than 0.1 mm
Much smaller than
0.1 mm
Glassy No grains at all; rock is a homogeneous mass of glass. —
FIGURE 4.2 Grain sizes in sedimentary rocks (a–d) and an igneous rock (e).
(a) Very fine grained.
(b) Fine grained.
1 cm
(c) Medium grained.
(d) Coarse grained.
(e) Glassy.
4.3 A ROCK IS MORE THAN THE SUM OF ITS MINERALS
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