How can you distinguish a clastic (fragmental) sedimentary rock from a chemically formed sedimentary rock?

Answer 1

Many sedimentary rocks are made from the broken bits of other rocks. These are called clastic sedimentary rocks. The broken bits of rocks are called sediment. Sediment is the sand you find at the beach, the mud in a lake bottom, the pebbles in a river, and even the dust on furniture. The sediment may, in time, form a rock if the little pieces become cemented together.

Unlike most other sedimentary rocks, chemical rocks are not made of pieces of sediment. Instead, they have mineral crystals made from elements that are dissolved in water.

The water in the oceans, lakes, and ground is often full of dissolved elements. All sorts of things can dissolve into water. If you put a spoonful of salt into water, the salt will eventually dissolve. Seawater tastes salty mainly because there are salty minerals such as halite dissolved in it.

Sometimes water becomes so full of dissolved elements that they will not all fit. Some are not able to remain dissolved and form solid mineral crystals. This usually happens when some of the water has evaporated away, leaving less room for the dissolved elements. If enough water evaporates, they do not all fit and some form crystals of minerals such as halite, gypsum, and calcite. In the picture to the left, minerals are forming out of shallow water that has flooded the bottom of Death Valley in California. The valley is so hot and dry that water evaporates very quickly, leaving behind the minerals that were once dissolved in it.

Answer 2

Clastic rocks are made up of broken bits and chunks of other rocks stuck together.
I don't know exactly what you mean by "chemically formed" since so many types of sedimentary rocks could be described as chemically formed, but if it's not a clastic rock, then it would not be made of pieces of other rocks stuck together.