Fossils can be found pyritised. Describe and explain how pyritisation occurs.

Pyritisation is the replacement of original material by iron pyrites. The environment needs to be anaerobic, where the only live organisms are sulfur bacteria. This bacteria will then use the sulfur in the environment to respire, reducing the sulfur to biosulfate. The biosulfate will react with any iron in the environment, to form the iron pyrites which then replace the fossil material.