Siderite Identification Guide
Identify siderite, the brown iron carbonate, using its rhombohedral cleavage, high density, weak acid reaction, and key differences from calcite and sphalerite.
Read the full Siderite encyclopedia entry →
What Siderite Looks Like
Siderite is iron carbonate (FeCO3), a common ore mineral. It typically appears in earthy yellow-brown to deep chocolate-brown tones, sometimes pale tan or gray when fresh, darkening as it oxidizes.
- Luster: vitreous to pearly, occasionally silky
- Transparency: translucent to nearly opaque
- Habit: rhombohedral crystals, frequently with curved or saddle-shaped faces; also botryoidal, granular, and massive earthy masses
- Surface clue: weathered siderite often develops a rusty-brown to nearly black oxidized crust
Step-by-Step Field-ID Checklist
- Heft it. Siderite feels notably heavy for a brown carbonate — denser than calcite.
- Look for curved rhombs. Saddle-shaped, curved crystal faces are a strong siderite clue.
- Find cleavage. Three directions of perfect cleavage (rhombohedral) produce blocky, leaning-rhomb fragments.
- Scratch test. It is soft — a steel knife scratches it (3.5–4.5).
- Acid test, the right way. Siderite barely fizzes in cold dilute HCl; warm the acid or powder the sample and it reacts more clearly, and the solution may turn yellow-green from dissolved iron.
Key Diagnostic Tests
- Mohs hardness: 3.5–4.5.
- Streak: white to pale yellowish (turns brown as it oxidizes).
- Cleavage: perfect rhombohedral (three directions, not at 90 degrees).
- Specific gravity: high, about 3.9 — heavier than calcite (2.7) or dolomite (2.85).
- Acid: slow/weak reaction with cold HCl; better when warmed or powdered.
- Magnetism: non-magnetic fresh, but becomes magnetic after heating/roasting (a confirmatory lab test).
Common Look-Alikes and How to Tell Them Apart
- Calcite: calcite fizzes vigorously in cold acid, is much lighter (SG 2.7), and is usually colorless/white; siderite is heavier, brown, and reacts only weakly.
- Dolomite: dolomite is lighter and only slightly reactive; siderite's high density (~3.9) and stronger brown color set it apart.
- Sphalerite: sphalerite shares a brown color and high SG but has a resinous/adamantine luster, dodecahedral cleavage, and a smell of sulfur (rotten egg) when scratched or acid-tested; siderite has no sulfur smell.
- Rhodochrosite: pink to rose tones and also a carbonate; color separates it, and siderite is browner.
- Limonite/goethite: these iron oxides have no rhombohedral cleavage, give a yellow-brown streak, and do not show carbonate crystal forms.
The combination of brown color + rhombohedral cleavage + high density + weak acid reaction is diagnostic.
Where Siderite Is Found
Siderite forms in sedimentary ironstones and bog/clay ironstone nodules, in hydrothermal veins with other ores, and in some pegmatites and carbonatites. Classic localities include the iron ranges of Europe (Austria's Erzberg, Germany, England's Cleveland ironstones), and many sedimentary basins worldwide where it occurs as concretions in shale and coal measures.
Frequently asked questions
How can you tell if it's real siderite?
Look for a heavy brown carbonate with perfect rhombohedral cleavage (often curved faces), hardness 3.5–4.5, a white streak, and only a weak reaction in cold acid that improves when the acid is warmed or the sample is powdered.
Why doesn't siderite fizz like calcite?
All carbonates react with acid, but siderite reacts slowly in cold dilute HCl. Warming the acid or powdering the mineral speeds the reaction, and the dissolving iron can tint the drop yellow-green.
Siderite vs sphalerite — how do I tell them apart?
Both can be brown and heavy, but sphalerite has a resinous luster, dodecahedral cleavage, and a sulfur smell when scratched or acid-tested, while siderite has rhombohedral cleavage and no sulfur odor.
Is siderite magnetic?
Fresh siderite is non-magnetic, but it becomes magnetic after being heated or roasted, which converts it toward magnetite — a useful confirmatory test.
Siderite identified by the community
Recent Siderite specimens identified with Rock Identifier.
