Goethite
Goethite (/ˈɡɜːrtaɪt/, US also /ˈɡoʊθaɪt/) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the α-polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient times for its use as a pigment (brown ochre). Evidence has been found of its use in paint pigment samples taken from the caves of Lascaux in France. It was first described in 1806 based on samples found in the Hollertszug Mine in Herdorf, Germany. The mineral was named after the German polymath and poet Johann Wolfgang von Goethe (1749–1832).
| Goethite | |
|---|---|
| General | |
| Category | oxide minerals hydroxide subgroup |
| Formula (repeating unit) | α-FeO(OH) |
| IMA symbol | Gth |
| Strunz classification | 4.FD.10 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H–M symbol: (2/m 2/m 2/m) |
| Space group | Pbnm |
| Identification | |
| Color | Yellowish to reddish to dark brown or black |
| Crystal habit | radial acicular, mammillary, botryoidal, stalactitic, massive, as encrustation, as pseudomorph; may be banded or iridescent |
| Cleavage | Perfect {010} |
| Fracture | Uneven to splintery |
| Mohs scale hardness | 5.0–5.5 |
| Luster | Adamantine to dull |
| Streak | Brown, brownish yellow to orange yellow |
| Specific gravity | 3.3–4.3 |
| Refractive index | Opaque to sub-translucent |
| Fusibility | Fusible at 5–5.5 |
| Solubility | HCl soluble |
| Other characteristics | weakly magnetic |
| References | |
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