When you melt a mixture of iron and sulfur, you get iron sulfide, which can form pyrite crystals. DEA/A.RIZZI, Getty Images. A mixture occurs when you combine matter in a way where the components can be separated again.A compound results from a chemical reaction between components, forming a new substance.hen you melt a mixture of iron and sulfur, you get iron sulfide, which can form pyrite crystals. DEA/A.RIZZI, Getty Images. A mixture occurs when you combine matter in a way where the components can be separated again.