Goethite

Goethite is a widespread iron oxyhydroxide and the principal crystalline mineral within the limonite mixture. It crystallizes in the orthorhombic system, forming slender prismatic and acicular (needle-like) crystals, but is far more often seen as botryoidal, stalactitic, reniform, or radiating fibrous masses with a satiny to submetallic luster.

Colors range from yellowish- and reddish-brown to nearly black, and it always gives a yellowish-brown to brownish streak. Goethite, named for the German poet Goethe, is essentially the mineral form of much natural rust.

Glossy black botryoidal goethite and "rainbow" iridescent goethite are popular among collectors, and velvety sprays of goethite needles enhance some quartz and amethyst specimens.

Goethite forms by the weathering and oxidation of iron-rich primary minerals such as pyrite, magnetite, siderite, and iron silicates, making it abundant in the surface zone of countless rocks and ore deposits.

It precipitates directly from iron-bearing waters in bogs, springs, and lake beds, and is a major constituent of laterite soils, gossans over sulfide ores, and the cement that colors many sandstones brown. Notable localities include Cornwall in England, the Lake Superior iron ranges, the Erzgebirge of Germany and the Czech Republic, and Morocco, which produces fine iridescent and botryoidal specimens.

Goethite is yellowish-brown to dark brown or black, with a satiny, silky, or submetallic luster and a diagnostic yellowish-brown streak that distinguishes it from red-streaked hematite. Hardness is around 5-5.5, harder than most earthy limonite.

Look for botryoidal, stalactitic, or radiating fibrous masses, sometimes with iridescent surfaces. It commonly forms pseudomorphs after pyrite and lines geode and vug cavities, sometimes within quartz. Distinguishing goethite from the broader limonite mixture relies on recognizing its crystalline, fibrous, or botryoidal habit; the streak test separates it from hematite and magnetite.

Goethite is an important iron ore, contributing heavily to many sedimentary iron deposits and laterites mined for iron and steel production. As the dominant phase in limonite, it underlies the use of bog iron and brown ores.

It is also a natural pigment, the basis of yellow and brown ochres used as artist's and industrial colorants. Botryoidal, stalactitic, and rainbow-iridescent goethite specimens are valued by collectors, and goethite inclusions add character to some quartz gemstones. Metaphysically it is associated with grounding and connection to earth, but its chief role is as ore and pigment.