Which of the following iron minerals is least stable in a humid temperate climate?

• A. Pyrite
• B. Hematite
• C. Magnetite
• D. Goethite

Answer: (A)

In humid temperate climates, chemical weathering is driven by oxidation, and sulfide minerals are especially prone to it. Pyrite contains iron bonded to sulfur, so it oxidizes readily when exposed to air and water. The oxidation quickly forms iron oxides/hydroxides and sulfuric acid, which accelerates breakdown of the mineral and surrounding rock. In contrast, iron oxides and related minerals like hematite and magnetite are far more stable under surface conditions, and goethite forms in moist environments but is still more chemically persistent than sulfide minerals. Because of this rapid, weathering-prone oxidation, pyrite is the least stable among the minerals listed.