Goethite
Goethite is FeO(OH), hardness 5 to 5.5, density 4.28, crystals to 45 cm. Its streak is ocher-yellow, and most material labelled limonite is in fact goethite.
Goethite is iron oxide-hydroxide, α-Fe3+O(OH), orthorhombic, hardness 5 to 5.5 with a measured density of 4.28 and prismatic crystals to 45 cm. The streak is brownish yellow to ocher-yellow. Most specimens labelled limonite in a mineral collection are goethite: limonite is a field term, not a species.
In short
- Limonite is not a mineral species. It is a field term for brown hydrous iron oxide of uncertain identity, and the Handbook of Mineralogy uses it in quotation marks for nearly cryptocrystalline goethite. Most limonite labels in a collection are goethite; some are a mixture with lepidocrocite or hematite.
- The streak identifies it in one drag. Goethite gives brownish yellow to ocher-yellow, hematite cherry-red, lepidocrocite dull orange, magnetite black. That is the fastest and cheapest determination available on an unlabelled brown or black iron mineral.
- Crystals reach 45 cm. Goethite is thought of as a botryoidal crust, but prismatic crystals striated along [001] are recorded to 45 cm, and tabular crystals on {010} occur. Crystallised goethite is a real collecting target, not a curiosity.
- It is trimorphous with feroxyhyte and lepidocrocite, all three being FeO(OH), so the species that look most like goethite have identical chemistry and different structures.
- Britain supplies both classic crystals and the reference material. The Handbook takes its X-ray powder pattern from the Hindlow quarry in Derbyshire, and names exceptional crystals from the Restormel mine at Lanlivery and the Botallack mine at St Just in Cornwall.
| Species | Formula | Streak | Hardness | Density | Other tell |
|---|---|---|---|---|---|
| Goethite | α-FeO(OH) | Brownish yellow to ocher-yellow | 5–5.5 | 4.28 | Perfect {010} cleavage; botryoidal with radial fibrous interior |
| Hematite | Fe2O3 | Cherry-red to reddish brown | 5–6 | 5.26 | No cleavage — parting only; noticeably heavier |
| Lepidocrocite | γ-FeO(OH) | Dull orange | 5 | 4.09 | Ruby-red to reddish brown; scaly, feathery habit |
| Magnetite | Fe3O4 | Black | 5.5–6.5 | 5.175 | Strongly magnetic; octahedra |
| Siderite | Fe2+CO3 | White | 3.75–4.25 | 3.96 | Much softer; effervesces in warm dilute acid |
| “Limonite” | Not a species | Yellow-brown | Variable | Variable | A field term. Determine it or label it as undetermined |
Why limonite is a label and goethite is a mineral
Limonite has no formula, no crystal system and no IMA status, because it is not a species. It is a nineteenth-century collective term for brown, earthy, hydrous iron oxide that could not be resolved further with the tools of the time. The Handbook of Mineralogy uses the word only in quotation marks, in the goethite entry, to describe material that is nearly cryptocrystalline goethite.
Modern work generally finds that such material is goethite, often with lepidocrocite, sometimes with hematite, adsorbed water and clay. So a limonite label is not wrong so much as unfinished. In a catalogue it should read as an undetermined identification rather than as a species name — the distinction is set out on labelling and cataloguing a collection.
This matters more than it sounds. A great deal of what sits in old British collections as limonite is in fact crystallised or botryoidal goethite with a genuine locality, and relabelling it correctly — as determined, by streak and habit, with the date — turns a vague specimen into a documented one without touching the mineral.
The streak test, and the three minerals it eliminates
A streak plate is unglazed porcelain at a hardness of about 6.5, which will mark everything in the brown-and-black iron group. Drag once, firmly, and look at the powder rather than at the scratch.
Goethite leaves brownish yellow to ocher-yellow powder. Hematite leaves cherry-red; our hematite page is built on that fact. Lepidocrocite leaves dull orange. Magnetite leaves black, and magnetite is also strongly magnetic, so the two observations together are conclusive.
The reason this matters beyond tidiness is that the same test is the fastest way to dispose of a supposed meteorite. Most rocks brought in as meteorites are hematite, goethite or magnetite, and all three streak. The elimination sequence is on is my rock a meteorite.
Where the streak is ambiguous — and it can be on mixed material — the remaining usable observations are cleavage and weight. Goethite has a perfect cleavage on {010}; hematite has none at all, only parting. Hematite at 5.26 is clearly heavier in the hand than goethite at 4.28 on comparable pieces. Our guide to identifying a mineral specimen covers how far to push that.
Crystallised goethite, and where it comes from
The species is a weathering product, and its common forms are reniform, botryoidal and stalactitic masses with a concentric or radial fibrous interior — the kidney-shaped material that fills any British collection of iron minerals. The crystals are the collecting prize. The Handbook records prismatic crystals striated parallel to [001] to 45 cm, tabular crystals on {010}, and aggregates of capillary to acicular crystals in divergent sprays.
The Cornish localities are the ones a British collector looks for: the Restormel mine at Lanlivery and the Botallack mine at St Just, both named in the Handbook for exceptional crystals. Elsewhere, Siegen in North Rhine-Westphalia and near Giessen in Hesse, Příbram in the Czech Republic, Chaillac in France, the Pikes Peak district and Florissant in Colorado, and the Lake Superior iron ranges at Negaunee and Marquette in Michigan.
There is a quiet British detail in the Handbook worth knowing. The X-ray powder pattern given for the species is from the Hindlow quarry, Derbyshire, England, with a 1975 Geological Survey of Great Britain bulletin as its reference. A Derbyshire quarry supplies the reference diffraction data for one of the commonest minerals on Earth. Our Derbyshire notes cover that orefield, and the species reference covers the iron minerals it produces.
Condition, cleaning and what goethite does in a collection
Goethite is chemically stable — it is already an oxidation product, so there is nothing left for it to oxidise into — and it does not need humidity control. That makes it one of the easier species to keep, with two caveats.
Botryoidal goethite is fragile in a specific way. The radial fibrous interior means that a knock can detach a whole botryoid along the growth boundary, leaving a clean crater. That damage is conspicuous and is a common undisclosed repair; the raking-light method on telling if a specimen is repaired finds it quickly.
And goethite stains everything it touches. Ocher-yellow powder transfers to tissue, foam and neighbouring specimens, so a goethite wants its own tray. The species is also the reason so many specimens of other minerals arrive with a rusty film: the guidance on cleaning mineral specimens covers when that film should be removed and when it is part of the specimen.
Goethite is named for Johann Wolfgang von Goethe, 1749 to 1832, the German poet, dramatist and philosopher, who was also a serious mineral collector. Well-crystallised Cornish material is always worth hearing about — the wanted list is where to tell us.