Goethite Identification Guide

What Goethite Looks Like

Goethite is an iron oxyhydroxide (FeO(OH)) and one of the most common iron-staining minerals on Earth. It ranges from yellowish-brown and ochre through dark brown to brownish-black. Well-crystallized specimens show a metallic to adamantine luster on fresh fracture, but most field material is dull, earthy, or silky.

Common forms include:

• Botryoidal and reniform masses with a smooth, kidney-shaped, often iridescent surface
• Radiating fibrous or velvety crystal sprays lining cavities
• Stalactitic and pisolitic crusts (the backbone of much "limonite" and bog iron)
• Pseudomorphs after pyrite (cubes now made of rusty goethite)

Step-by-Step Field ID Checklist

1. Note the color and surface. Brown to black, with a smooth botryoidal or fibrous internal structure when broken.
2. Streak it. This is the key test: goethite leaves a distinctive yellowish-brown to ochre streak on an unglazed tile.
3. Check hardness. Crystalline goethite is 5–5.5; earthy masses feel much softer and crumbly.
4. Look at the break. Crystalline goethite shows a silky fibrous to splintery fracture; massive material is uneven and earthy.
5. Test magnetism. Goethite is essentially non-magnetic (weakly paramagnetic), unlike magnetite.
6. Weigh it in hand. Specific gravity is ~3.3–4.3, noticeably heavy but lighter than hematite or magnetite.

Key Diagnostic Tests

• Streak: yellow-brown / ochre — the single most reliable separator from hematite.
• Hardness: 5–5.5 (crystalline); scratches glass with effort.
• Magnetism: none to very weak (distinguishes from magnetite).
• Acid: does not effervesce in dilute HCl (rules out siderite and carbonates).
• Habit: botryoidal/fibrous internal banding under a loupe is highly characteristic.

Common Look-Alikes and How to Tell Them Apart

• Hematite: very similar brown-black exterior, but hematite gives a red to red-brown streak. Streak test settles it instantly.
• Magnetite: black, black streak, and strongly magnetic — goethite is not.
• Limonite: not a true mineral but a field term for amorphous hydrated iron oxides; much of it is cryptocrystalline goethite. If it shows fibrous structure and a 5+ hardness, call it goethite.
• Siderite: brown carbonate that fizzes in warm dilute HCl; goethite does not.
• Pyrolusite/psilomelane (Mn oxides): softer, with a black streak rather than ochre.

Where Goethite Is Found

Goethite forms by weathering and oxidation of iron-bearing minerals, so it is virtually everywhere iron rusts. Look for it as gossan capping over sulfide ore bodies, as bog iron in swamps and lakes, in laterite soils, and as botryoidal masses lining vugs in iron-rich rocks. Famous specimens come from Colorado (Pikes Peak), the Lake Superior iron ranges, Cornwall, and Germany's Harz Mountains.