Why is mudstone grey or black?

Question

Hi this is a homework quetsion, any ideas??

Answer 1

It depends on the amount of organic matter. The higher the organic matter it has, the darker the color.

Answer 2

The colour depends on the original mineralogy of the silts/muds that make up the deposit. Some silts/muds are naturally darker than others. such as Illite (dark) kaolinite (light). The colour has very little to do with organic matter because little original organic matter is left after compaction and lithofication. Sometimes you can get fossils in these types of rocks but they are usually calcified or salicified.