Examine the grain card only (picture of grain size chart given below). Which grain size category are the smallest grains you can see with your naked eye? It is OK for this to be slightly different than other people. Explain. Also, what grain size, in mm, does this correspond to?
Clastic grains that are greater than 2 mm are referred to as gravels—divided into granules (2–4 mm), pebbles (4–6 mm), cobbles (64– 256 mm), and boulders (>256 mm) and the rocks that form from them are called conglomerates or breccias, depending on whether most of the clasts are mostly rounded or angular, respectively. These are easy to see with the naked eye. The chart to the left is not very useful for these grain sizes.
Clastic grains that are between 1/16 mm and 2 mm are referred to as sand and the rocks that contain such grain sizes are called sandstones. Sands and sandstone grains can be divided thus:
Very fine sand = 63–125 µm
Fine sand = 125– 250 µm
Medium sand = 250–500 µm
Coarse sand = 500 µm–1 mm
Very coarse sand = 1–2 mm
Grain sizes that are smaller than 1/16 mm are referred to as silt (1/256–1/16 mm, also written as 39–63 µm) or mud/clay (<1/256 mm which is the same as <39 µm). (They are written in both formats for historical reasons.) Because these grain sizes are so small, they’re hard to tell apart without using a microscope. However, silt grains tend to feel a little ‘rougher’ between the fingertips. Sedimentary rocks made of these grain sizes are referred to as siltstones or mudstones/claystones, accordingly.
Question:
- Examine the grain card only (picture of grain size chart given below). Which grain size category are the smallest grains you can see with your naked eye? It is OK for this to be slightly different than other people. Explain. Also, what grain size, in mm, does this correspond to?
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