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Limonite

Limonite Stalactite

Limonite is scientifically not considered a true mineral as it lacks a definitive chemical formula and crystal structure. However, all mineral reference guides list it together with other minerals. Limonite is a matrix base of many other minerals, and the term gossan is used as a reference to Limonite when it is used as a a matrix for another mineral or has formed an undesirable staining on top of it.

Limonite is extremely common and forms the coloring matter in many soils. It is also responsible for the coloring on the weathered surfaces of rocks. Much Limonite, especially the fibrous type, is either Goethite or altered Goethite that absorbed water in its chemical structure.

Limonite commonly forms as a pseudomorph over other minerals, especially Pyrite, Marcasite and Siderite. These pseudomorphs are very identifiable by their partial rusting look and yellow staining.

Chemical Formula

FeO(OH) · nH₂O

Color

Yellow, brown, reddish-brown. Occasionally iridescent in a play of colors.

Crystal System

Amorphous

Properties

Streak
Yellow to brown

Hardness
4 - 5.5

Transparency
Opaque

Specific Gravity
2.7 - 4.5

Luster
Dull, vitreous, or silky

Cleavage
None

Fracture
Splintery, uneven

Tenacity
Brittle

Crystal Habits

Limonite does not form in crystals. It occurs amorphous in earthy, massive, botryoidal, mammilary stalactitic, and fibrous form. May also form as a pseudomorph after other minerals, especially Pyrite, Siderite, and Marcasite.

Additional Information

Composition
Hydrous basic iron oxide, with an indefinite composition

In Group
Oxides; Hydroxides

Striking Features
Color, streak, and formations

Environment
As a secondary mineral formed from the alteration of iron minerals.

Other Names

	Bog Iron	Synonym used for both Goethite and Limonite.
	Bog Iron Ore	Synonym used for both Goethite and Limonite.
	Brown Iron Ore	Synonym used for both Goethite and Limonite.
	Wood Iron	Synonym used for both Goethite and Limonite.

Uses

Limonite in a ore of iron and is used as a pigment in the manufacture of paints. Besides for a few aesthetic lustrous botryoidal forms of Limonite, as well as the interesting pseudomorphs, Limonite is not of much interest to collectors.

Noteworthy Localities

Limonite is extremely common and found worldwide. Excellent iridescent botryoidal masses have recently been found in China at Nandan, Guangxi Province, and at Qinglong, Guizhou Province. Excellent Limonite pseudomorphs after Pyrite come from Rio Marina, Elba Island, Italy; and Arenalejos, Carratraca, Spain.

In the U.S., pseduomorphed Pyrite comes from Fruitville, Lancaster Co., Pennsylvania; Aquadale and Porter, Stanly Co., North Carolina; Pelican Point, Utah Co., Utah; and the Elizabeth R. Mine, Pala, San Diego Co., California.