The mineral is Iron Sulphide. It is not a mixture, but a compound. A mixture is when two substances are together, but not chemically bonded. For example, if I add food colouring to water, I have made a mixture because the food colouring has not chemically bonded to the water. If I react Iron with Suphur, the resulting substance is a compound. The resulting mineral will not burn or be magnetic because the compound has different properties to the elements themselves. For example, chlorine gas is toxic and sodium reacts violently with water, but sodium chloride is table salt, nor toxic nor explosive.
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