Goethite is a common weathering product of iron-bearing minerals like magnetite, pyrite and siderite. It is an oxyhydroxide or iron — FeO(OH). The most characteristic feature of this mineral is brown to yellowish color. It may resemble hematite (Fe₂O₃), but hematite is either gray or reddish. It changes to hematite on dehydration. Powdered hematite is reddish brown, but goethite is yellowish brown. Both of these minerals are good paint pigments. Goethite is the main constituent of yellow ochre.

Oolite consisting of goethitic ooids from Germany. Width of sample 12 cm.

It is a common iron-bearing mineral in soil. It is usually formed under oxidizing conditions. It precipitates directly from marine and meteoric waters and is the main mineral phase containing precipitated iron in these environments. It may form mixtures with other similar minerals which may alter the color. Limonite is a mixture of several iron oxides and hydroxides, goethite usually being one of the most important constituents. It may be mined as a mineral resource. Bog iron is mostly composed of it. Goethite is the principal component in sedimentary iron ore in the Lorraine Basin of France¹.

The mineral is named after the German poet Johann Wolfgang von Goethe who was also interested in mineralogy.

Sand sample from Martha’s Vineyard, Massachusetts, USA. Brown rounded pellets are goethite grains. Width of view 10 mm.

Bog iron is mostly composed of it. Width of sample from Estonia is 14 cm.

Hematitic iron ore (SEDEX-type) with goethite (yellow). Svinsås, Norway. Width of sample 13 cm.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.