Siderite
Siderite is iron carbonate, FeCO3: hardness 3.75 to 4.25, density 3.96, perfect rhombohedral cleavage, 48.20% iron. How to tell it from ankerite by weight.
Siderite is iron carbonate, FeCO3 (IMA: Fe(CO3)), a calcite-group mineral: hexagonal, hardness 3.75 to 4.25, measured density 3.96, perfect rhombohedral cleavage, white streak, usually yellowish brown to brown. It is 48.20% iron by mass, and the Handbook calls it a common component of bedded sedimentary iron ores.
In short
- Iron carbonate, worked. FeCO3 has a formula mass of 115.85 on standard atomic weights: 48.20% iron, 62.01% FeO and 37.99% CO2, which is the ideal column on the Handbook of Mineralogy sheet. The sheet’s own analysis, from Camborne in Cornwall, gives 61.08% FeO, or 47.48% iron; iron is 97.6% of its metal cations, manganese 1.8%.
- The 14.25 g line for brown rhombs. A 20.00 g piece weighs 14.95 g in water as siderite (3.96) and 13.17 to 13.55 g as ankerite (2.93 to 3.10), its brown look-alike. Halfway between is 14.25 g, a specific gravity of about 3.48. Of the seven other carbonates in the table, five fall below the line and two, rhodochrosite and smithsonite, above it.
- Where the line fails. Rhodochrosite reads 14.59 g, only 0.35 g from siderite, and its sheet includes cinnamon-brown. Siderite forms series with rhodochrosite, magnesite and smithsonite, so an intermediate piece reads in between. The scale rules ankerite and dolomite out; it does not prove siderite.
- The sheet records no fluorescence and no magnetism. On a laboratory separator, USGS Open-File Report 99-529 pulled siderite at 0.10 to 0.50 amp across 17 samples, the report’s paramagnetic range. The full magnet question is on which minerals are magnetic.
- British localities are Cornish and Devonian on the sheet. It names the Great Onslow Consols mine, Wheal Maudlin and Dolcoath in Cornwall and the Virtuous Lady mine, Tavistock, Devon. It does not name Weardale.
| Species | IMA formula | Group on the sheet | Hardness | Density (measured) | 20.00 g piece in water | Colour on the sheet (selection) | Side of the 14.25 g line |
|---|---|---|---|---|---|---|---|
| Siderite | Fe(CO3) | Calcite group | 3.75 to 4.25 | 3.96 | 14.95 g | Yellowish brown, brown, white, ash-grey, pale green | Above |
| Smithsonite | Zn(CO3) | Calcite group | 4 to 4.5 | 4.43 | 15.49 g | White, greys, pale brown, brown; rarely pink, green, blue | Above |
| Rhodochrosite | Mn(CO3) | Calcite group | 3.5 to 4 | 3.70 | 14.59 g | Pink, rose-red, cherry-red, yellow, cinnamon-brown | Above |
| Kutnohorite | CaMn(CO3)2 | Dolomite group | [3.5 to 4] | 3.12 | 13.59 g | White, pale pink to bright pink | Below |
| Ankerite | CaFe2+(CO3)2 | Dolomite group | 3.5 to 4 | 2.93 to 3.10 | 13.17 to 13.55 g | Brown, yellow, white | Below |
| Magnesite | Mg(CO3) | Calcite group | 3.5 to 4.5 | 3.00 | 13.33 g | Colourless, white, pale yellow, pale brown | Below |
| Dolomite | CaMg(CO3)2 | Dolomite group | 3.5 to 4 | 2.86 | 13.01 g | Colourless, white, yellow, brown, pale pink | Below |
| Calcite | Ca(CO3) | Calcite group | 3 | 2.7102 | 12.62 g | Colourless or white, many others | Below |
What is siderite? An iron carbonate of the calcite group
The names. The September 2026 IMA-CNMNC Master List lists siderite as Fe(CO3), status A, 1962 s.p., with the country of first description given as unknown. The Handbook sheet writes Fe2+CO3 and files it under Calcite group. The name, in the sheet’s words, is from the Greek sideros, for iron in the composition.
What it looks like. Commonly crystallised: rhombohedral {10-11} to steep scalenohedral {21-31}, prismatic {10-10} and {0001}, to 25 cm. It is also fibrous, stalactitic, spherulitic, cleavable and fine-grained massive. Colour runs yellowish brown and brown through white, ash-grey and pale green, but the streak is white whatever the colour; the method is on how to use a streak plate. Cleavage is perfect on {10-11}, in three directions, like calcite. Twinning is uncommon, lamellar on {01-12}.
Where it forms. The sheet’s Occurrence field: a common component of bedded sedimentary iron ores and metamorphic iron formations; in hydrothermal metallic veins; rarely in granite and nepheline syenite pegmatites; in carbonatites; authigenic, and in concretions. Its listed associates are quartz, barite, fluorite and pyrite.
Is siderite an iron ore? Iron by mass, worked
The arithmetic, on standard atomic weights (Fe 55.845, C 12.011, O 15.999). FeCO3 has a formula mass of 115.85. Iron is 55.845 of that: 48.20%. Expressed as oxides it is 62.01% FeO and 37.99% CO2, the sheet’s ideal column to the hundredth. The live which minerals are magnetic page uses the same 48.20%.
On a real analysis. The sheet’s Camborne, England analysis gives CO2 38.19, FeO 61.08, MnO 1.12, MgO 0.13 and CaO 0.10, total 100.62. That is 47.48% iron by mass. Dividing each oxide by its formula mass, iron is 97.6% of the metal cations, manganese 1.8%, magnesium 0.4% and calcium 0.2%: siderite, close to the end member.
Against the iron oxides, per kilogram. Worked the same way: siderite carries 482 g of iron per kg, goethite 629 g, hematite 699 g and magnetite 724 g. The Handbook calls siderite a common component of bedded sedimentary iron ores; it does not rank it as an ore, and neither do we. The oxides have their own pages: hematite, goethite and magnetite.
Siderite or ankerite? The 14.25 g line
Why it is needed. Ankerite’s sheet gives brown, yellow and white, a white streak, hardness 3.5 to 4 and perfect rhombohedral cleavage on {10-11}: in the hand, a brown rhomb either way. Hardness overlaps at 3.75 to 4. Density does not: 3.96 against 2.93 to 3.10.
The rule. Weigh a clean 20.00 g piece in air, then suspended in water. Siderite reads 14.95 g; ankerite 13.17 to 13.55 g, at least 1.40 g lighter. Take the midpoint, 14.25 g, a specific gravity of about 3.48. Below it, the piece is not siderite; ankerite, dolomite (13.01 g), magnesite (13.33 g) and kutnohorite (13.59 g) all fall there. The method, with its error sources, is on how to measure specific gravity at home.
Documented failures. Above the line the scale stops helping. Rhodochrosite, 14.59 g, is 0.35 g from siderite, and its sheet lists cinnamon-brown. Smithsonite, 15.49 g, is 0.54 g above, and its sheet lists brown too. Siderite forms series with rhodochrosite, magnesite and smithsonite, so manganese- or magnesium-rich material reads between the end members. A crust on matrix measures the matrix. The scale says not ankerite, not siderite; rhodochrosite and magnesite have their own pages.
Where siderite is found: Cornwall, Devon, Panasqueira and Bisbee
The Distribution field opens many noted localities worldwide and names: Freiberg and Neudorf, Harz Mountains, and the Siegerland district, Germany; the Erzberg near Eisenerz, Styria, and Hüttenberg-Lölling, Carinthia, Austria; Allevard, France; Panasqueira, Portugal; Ivigtut, Greenland; very large crystals at Mont Saint-Hilaire, Quebec; Bisbee and the Antler mine, Arizona; Leadville, Colorado; large crystals from Mosojllacta, Colavi, Bolivia; and the Morro Velho gold mine, Nova Lima, and Pedreira Ataleia, Governador Valadares, Minas Gerais.
In England the sheet says many mines in Cornwall, naming the Great Onslow Consols mine, St. Breward; Wheal Maudlin, Lanlivrey (the sheet’s spelling); and the Dolcoath mine, Camborne. In Devon it names the Virtuous Lady mine, Tavistock. The sheet does not name Weardale or any other northern English field, and we add none.
The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.