Discussion
Mark the correct statement regarding deposition of pyrite :
Answer: D
The full statement is that pyrite is deposited in a reducing environment under both acid and alkaline conditions.
Pyrite (FeS2) needs sulphide, which is produced only where oxygen is absent, typically by bacteria reducing sulphate in organic-rich mud. On an Eh-pH diagram, such as the fence diagram of Krumbein and Garrels, its field lies at low Eh and extends across the neutral line, so it forms on both the acid and the alkaline side.
Option A is true as far as it goes but leaves out the pH range that the paper is testing, so the complete statement D is preferred. Option B says nothing about the redox condition, which is the decisive control. Option C is wrong: in an oxidising environment iron sulphide is not deposited but destroyed, its iron going to oxides and hydroxides.
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