Dolomite
Dolomite is CaMg(CO3)2: hardness 3.5 to 4, density 2.86, rhombohedral cleavage, saddle-shaped aggregates. Where dolomite ends and ankerite begins, worked.
Dolomite is calcium magnesium carbonate, CaMg(CO3)2, a rock-forming mineral: hexagonal, hardness 3.5 to 4, measured density 2.86, perfect rhombohedral cleavage, colourless, white, brown or pale pink with a vitreous to pearly lustre and saddle-shaped aggregates among its habits. Where iron outweighs magnesium the species is ankerite instead; the line falls at 17.95% FeO.
In short
- The mineral, on both lists. CaMg(CO3)2 on the September 2026 IMA-CNMNC Master List and on the Handbook of Mineralogy sheet, grandfathered, 1792, Italy. It is a separate species from calcite, not a variety of it: it carries magnesium in its formula and has its own group, the dolomite group.
- The ankerite line, worked. Ankerite is CaFe2+(CO3)2. Where iron and magnesium are equal, CaMg0.5Fe0.5(CO3)2 is 17.95% FeO and 10.07% MgO by mass. Below 17.95% FeO with only iron and magnesium besides calcium, the species is dolomite; above it, ankerite. The sheet’s Haley, Ontario analysis has 0.22% FeO: magnesium is 99.4% of the pair.
- Iron shows on a magnetic separator. USGS Open-File Report 99-529 did not pull dolomite at 1.70 amp, its ceiling, across 4 samples. Iron-bearing dolomite, which it lists as ferrodolomite, was pulled from 0.20 amp, and ankerite at 0.30 to 0.70. The report states that iron-bearing carbonates are more magnetic than iron-free ones.
- Saddles, pearly lustre and curved faces are on the sheet. It records crystals with curved faces, saddle-shaped aggregates and a vitreous to pearly lustre. It also records that dolomite may fluoresce white to pink under UV and is triboluminescent; is calcite UV reactive compares the calcite-group sheets.
- Named for Dolomieu. The sheet honours Déodat de Dolomieu (1750–1801), French geologist, who contributed to early descriptions of the species in dolostone, the rock. A British locality is on the sheet: Frizington, west Cumbria.
| Species | IMA formula | Handbook formula | Hardness | Density (measured) | 20.00 g piece in water | USGS best range (amp); samples | Iron in the IMA formula? |
|---|---|---|---|---|---|---|---|
| Dolomite | CaMg(CO3)2 | CaMg(CO3)2 | 3.5 to 4 | 2.86 | 13.01 g | Above 1.70; 4 | No |
| Ferrodolomite (USGS name; not an IMA row) | None | None | Not on a sheet | Not on a sheet | Not worked | 0.40 to 1.10, depends on chemistry; 8 | Not applicable |
| Ankerite | CaFe2+(CO3)2 | Ca(Fe2+,Mg,Mn2+)(CO3)2 | 3.5 to 4 | 2.93 to 3.10 | 13.17 to 13.55 g | 0.40 to 0.50; 1 | Yes |
| Kutnohorite | CaMn(CO3)2 | Ca(Mn2+,Mg,Fe2+)(CO3)2 | [3.5 to 4] | 3.12 | 13.59 g | Not tabled | No |
| Calcite | Ca(CO3) | CaCO3 | 3 | 2.7102 | 12.62 g | Above 1.70; 8 | No |
| Magnesite | Mg(CO3) | MgCO3 | 3.5 to 4.5 | 3.00 | 13.33 g | Above 1.70; 2 | No |
Is dolomite a mineral or a rock? Both names are in use
The mineral is CaMg(CO3)2, a row on the September 2026 IMA-CNMNC Master List, status G, 1792, with Italy as the country of first description. The Handbook sheet calls it a major rock-forming mineral, abundant worldwide with numerous commercial uses. It does not list the uses, and this page does not either.
The rock. The sheet’s Name field says Dolomieu contributed to early descriptions of the species in dolostone, the term it uses for the rock. A label reading dolomite on a grey, granular block may mean the rock; on a pale pink saddle of curved rhombs it means the mineral. Crystal vs mineral sets out the general distinction.
How it forms, per the sheet: by diagenesis or hydrothermal metasomatism of limestone; as a primary phase in hypersaline sedimentary environments; as a major component of some contact metamorphic rocks and marbles; as a gangue in hydrothermal veins; and in carbonatites and ultramafic rocks. The association lists change with the setting: fluorite, barite, calcite, siderite, quartz and metal sulfides in hydrothermal veins; talc, serpentine, magnetite, forsterite and others in metamorphic rocks.
Dolomite or ankerite? The 17.95 per cent line, worked
The principle: name it for the dominant cation at the magnesium site. The IMA formulas differ in one place: dolomite CaMg(CO3)2, ankerite CaFe2+(CO3)2, kutnohorite CaMn(CO3)2. The Handbook writes ankerite as Ca(Fe2+,Mg,Mn2+)(CO3)2, and its sheet notes that ankerite was regarded as a variety of dolomite by many older authors, so an old label reading dolomite may be either.
The arithmetic, on standard atomic weights. Ideal dolomite, formula mass 184.40, is 30.41% CaO, 21.86% MgO and 47.73% CO2, the sheet’s ideal column. Ideal ankerite, 215.94, is 33.27% FeO. At the midpoint, CaMg0.5Fe0.5(CO3)2, formula mass 200.17, the composition is 17.95% FeO and 10.07% MgO. In a calcium carbonate with only magnesium and iron at the second site, below 17.95% FeO is dolomite and above it ankerite. Once manganese enters, compare cation proportions instead; that calculation for kutnohorite is on the kutnohorite page.
Worked on the sheet’s analysis. Haley, Ontario: CaO 31.27, MgO 21.12, FeO 0.22. Dividing by formula masses, magnesium is 99.4% of magnesium plus iron, and calcium to (magnesium plus iron) is 1.06 to 1. That is dolomite, very near the end member. The scale cannot do this: ankerite’s 13.17 to 13.55 g in water sits only 0.16 g or more above dolomite’s 13.01 g for a 20.00 g piece.
Is dolomite the same as calcite? Two species, one shape
No. Calcite is Ca(CO3), calcite group, hardness 3, density 2.7102; dolomite is CaMg(CO3)2, dolomite group, hardness 3.5 to 4, density 2.86. Both cleave perfectly on {10-11} into rhombs, so cleavage does not separate them. The live is aragonite the same as calcite page sets out the rhombs-then-density rule that separates calcite, dolomite and aragonite; it is not repeated here.
Structure. The sheet gives dolomite point group 3 and space group R3, against calcite’s point group 3 2/m. Twinning is common, as simple contact twins with {0001}, {10-10} or {11-20} as twin plane, and lamellar on {02-21}, where the sheet also records a parting.
A documented negative on the magnet. USGS lists both calcite and dolomite as not pulled at 1.70 amp, so a magnetic separator does not separate pure dolomite from calcite either. It does flag iron: the report gives iron-bearing dolomite a range from 0.20 amp. Neither the report nor the Handbook tests a hand magnet; the background is on which minerals are magnetic.
Where fine dolomite is found, including Cumbria and Naica
The Distribution field names, for fine examples: Traversella and Brosso, Piedmont, Italy; exceptional crystals from Eugui, Navarra, Spain; Trieben and Hall, Tirol, Austria; Freiberg and Schneeberg, Saxony; Lengenbach, Binntal, Switzerland; Trepča, Serbia; the Vuoriyarvi carbonatite complex, Kola Peninsula, Russia; fine crystals from Brumado, Bahia, and the Morro Velho gold mine, Nova Lima, Minas Gerais, Brazil; and Naica, Chihuahua, Mexico.
In Britain the sheet names one place, Frizington, Cumbria, in the west Cumbrian hematite field. In the USA it names Lockport, Niagara Co., eastward to Walworth, New York; Stony Point, North Carolina; and the Tri-State district at Joplin, Missouri, Galena, Kansas, and Picher, Oklahoma. We add no locality the sheet does not name.
Siderite, its frequent vein companion on the association list, has its own page at siderite. 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.