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n. An exposed, oxidized portion of a mineral vein, especially a rust-colored outcrop of iron ore.

n. decomposed rock, usually reddish or ferruginous (owing to oxidized pyrites), forming the upper part of a metallic vein

n. Decomposed rock, usually reddish or ferruginous (owing to oxidized pyrites), forming the upper part of a metallic vein. Called also iron hat.

n. In mining, the ferruginous quartzose material which often forms a large part of the outcrop of a lode in which the metallic contents at depths exist chiefly in the form of sulphids, among which pyrites, a combination of sulphur and iron, is rarely wanting, and is often present in large quantity.

"Under weathering conditions pyrite oxidizes, the sulphur forming sulphuric acid, -- an important agent in the secondary enrichment of copper and other sulphides, -- and the iron forming the minerals hematite and limonite in the shape of a "gossan" or "iron-cap.""

iron hat