goethite

n.
A red, yellow, or brown mineral, essentially HFeO₂, one of the common constituents of rust.

[After Johann Wolfgang von GOETHE.]

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A mineral of composition FeO · OH, crystallizing in the orthorhombic system. Crystals are rare, and the mineral is usually in reniform or stalactitic masses which have a radiating fibrous internal structure. The luster is adamantine to dull, and the color light to dark brown. The Mohs hardness is 5.0–5.5, and the density is 4.28 for crystals and 3.3–4.3 for massive material. Most of the common, yellow-brown, earthy ferric oxides known as limonite are mixtures composed largely of cryptocrystalline goethite.

Goethite is one of the most common minerals. It is the major constituent of the gossan at the surface of metalliferous deposits rich in iron-bearing sulfides, as at Bisbee, Arizona, and of laterites, as in Cuba. Well-formed crystals are found at Pribram, Bohemia, and Cornwall, England. It is an important iron ore in Alsace-Lorraine, in the Lake Superior hematite deposits, and in the southern Appalachians. See also Limonite.

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HFeO
Orthorhombic -- Rhombic bipyramidal

Environment

Secondary oxidized deposits (with rare exceptions the stuff of rust, the limonite of gossans); sometimes in crystals in low temperature veins.

Crystal description

Small, black, shiny, equidimensional crystals rare. Commonly in slender flattened plates, velvety surfaces of needles, and occasionally in brilliant rosettes of radiating plates. Also fibrous-massive with reniform surfaces; compact or earthy, even vitreous in the type called Glaskopf. Compare with limonite (following). Golden needles included in quartz.

Physical properties

Brilliant black to brownish black (crystals) to brown to yellow (fibrous varieties). Luster adamantine-metallic, glassy to silky and matte; hardness 5-5Ɖ; specific gravity 3.3-4.3; streak brownish yellow to yellow; fracture uneven; cleavage side pinacoid of bladed crystals. Brittle to crumbly, sometimes glassy; translucent brown or yellow in thin splinters.

Composition

Hydrogen iron oxide (62.9% Fe, 27.0% O, 10.1% H ₂ O).

Tests

Gives off water in closed tube and turns to hematite and, with time and heat, becomes magnetic. Practically infusible on charcoal.

Distinguishing characteristics

Distinguished from hematite by its streak and from limonite by its structure (silky, fibrous, radiating). The magnetism after heating distinguishes it from most other similar minerals.

Occurrence

After hematite, goethite is the most important ore of iron. Many substances formerly regarded as limonite are now recognized as having a definite goethite structure. In veins it forms crystals in the late stages, and thus becomes an accessory mineral of ore deposits (fluorite, barite, and hematite). Also (and more important economically), it is a secondary mineral formed under weathering conditions from sulfides and siderite. It is deposited as "bog iron ore," and forms residual brown iron ores in the sw. U.S., in Missouri, and in Cuba.

Widespread in Germany, France, and England in crystallized vein specimens and often in pseudomorphs after concretionary pyrite crystals. In the U.S., the best specimens are radiating crystal clusters from pegmatite pockets of the Florissant region of Colorado. Good fibrous specimens are found in the iron mines of Michigan and Minnesota. Often seen as tiny brownish tufts on and in quartz crystals, druse veins, and quartz-crystal-lined geodes.

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