Limonite Identification Guide

What Limonite Looks Like

Limonite is not a single mineral but a field name for a mixture of hydrated iron oxide-hydroxides (largely goethite) that forms by weathering of iron-bearing minerals — essentially natural rust. It ranges from earthy ochre masses to botryoidal (grape-like) crusts and is the main coloring agent in yellow-brown soils, bog iron, and "gossan" iron caps over ore bodies.

• Color: yellow-brown, ochre, dark brown to nearly black
• Luster: dull and earthy to silky or submetallic on botryoidal surfaces
• Transparency: opaque
• Habit: massive, earthy, botryoidal, stalactitic, or as pseudomorphs after pyrite
• Streak: yellow-brown — the standout diagnostic feature

Step-by-Step Field-ID Checklist

1. Do the streak test. Rub it on unglazed porcelain — limonite leaves a yellowish-brown streak. This is the most reliable identifier and separates it from hematite (red-brown streak) and magnetite (black streak).
2. Note the color and earthy feel. Ochre-brown, often staining your fingers, points to limonite.
3. Check the habit. Botryoidal crusts, earthy masses, and cubic pseudomorphs (boxwork after pyrite) are typical.
4. Test for magnetism. Limonite is essentially non-magnetic — distinguishing it from magnetite.
5. Assess weight. It is moderately heavy but lighter and softer-feeling than dense hematite or magnetite.

Key Diagnostic Tests

• Streak: yellow-brown to ochre — diagnostic.
• Mohs hardness: variable, roughly 4–5.5 (earthy forms feel softer; compact goethitic forms harder).
• Specific gravity: about 2.7–4.3, depending on water content and compactness.
• Magnetism: non-magnetic (unless contaminated). If strongly magnetic, it is magnetite, not limonite.
• Acid: little to no reaction.
• Fracture: earthy/uneven; no cleavage.

Common Look-Alikes and How to Tell Them Apart

• Hematite: the key comparison. Hematite gives a red-brown (cherry) streak, while limonite gives a yellow-brown streak — the streak test settles it.
• Goethite: chemically the dominant component of limonite; well-crystallized goethite gives a brownish-yellow streak too, and the distinction is largely about crystallinity. "Limonite" is the catch-all for poorly crystalline material.
• Magnetite: black streak and strongly magnetic — easy to separate.
• Bog iron / ironstone: these often ARE limonite-rich rocks; the term overlaps.
• Yellow ochre / clay: softer and more powdery, but composition overlaps with earthy limonite.
• Pyrite: brassy metallic with a greenish-black streak; weathered pyrite, however, commonly alters TO limonite, sometimes preserving cube shapes.

Where It Is Typically Found

Limonite forms wherever iron minerals weather in the presence of water and oxygen: in soils, swamps and bogs (bog iron), as gossans capping sulfide ore deposits, in laterites, and as cement and staining in sandstones. It is essentially ubiquitous in weathered iron-rich terrains worldwide and was historically an important iron ore.