Is dolomite a limestone?
Is dolomite a limestone? Not in the BGS scheme: limestone is mostly calcite, dolostone mostly dolomite, a line at 10.93% MgO, worked. The mineral and the rock.
Not in the British Geological Survey scheme. Dolomite is a mineral, CaMg(CO3)2, and a rock made mostly of it is a dolostone, formerly called dolomite; limestone is a rock whose carbonate is mostly calcite or aragonite. The BGS line falls at 50% dolomite in the carbonate, 10.93% MgO by mass in a calcite–dolomite rock.
In short
- The verdict. The BGS Rock Classification Scheme entry for limestone defines limestone as sedimentary rock with components over 50% calcite or aragonite. Its dolostone entry: over 50% magnesium carbonates, over half of that dolomite, “Formerly dolomite.” So a rock that is mostly dolomite is a dolostone, not a limestone. The word dolomite now names the mineral, CaMg(CO3)2 on the September 2026 IMA-CNMNC Master List.
- The BGS naming ladder. BGS Research Report RR 99-03 uses dolomitic as a qualifier “where dolomite makes up 10 to 50 % of carbonate within a limestone”, and calcareous, or more precisely calcitic, where calcium carbonate makes up 10 to 50% of a dolomite rock. So: under 10% dolomite, limestone; 10 to 50%, dolomitic limestone; 50 to 90%, calcitic dolostone; over 90%, dolostone.
- The MgO lines, worked. Pure dolomite is 21.86% MgO and calcite none. In a rock of the two alone, dolomite’s share of the carbonate by mass is MgO ÷ 21.86, so the 10%, 50% and 90% lines fall at 2.19%, 10.93% and 19.67% MgO. A chemical analysis of the rock, not the hand specimen, is what places it.
- Documented failure: mass or volume? The BGS percentages do not say whether they are by mass or by volume. If they are by volume, the 50% line moves to 11.22% MgO, because dolomite (2.86) is denser than calcite (2.7102). And magnesium in another mineral, such as magnesite or clay, would push MgO up without any dolomite, so the conversion holds only for a calcite–dolomite rock.
- Marble, mineral, sedimentary rock. Dolomite is a mineral with its own IMA row. Dolostone is a sedimentary rock in the BGS scheme. Metamorphosed, BGS calls it a dolomitic metacarbonate-rock and treats marble as a field term, misused for some stones such as Purbeck Marble. The Dolomites as a place are out of scope.
| Dolomite share of the carbonate | BGS name | Source wording | MgO in the rock, by mass basis | MgO if the share is by volume | Is it a limestone? |
|---|---|---|---|---|---|
| Under 10% | Limestone | Over 50% calcite/aragonite (RCS LMST) | Under 2.19% | Under 2.29% | Yes |
| 10 to 50% | Dolomitic limestone | “dolomite makes up 10 to 50 % of carbonate within a limestone” (RR 99-03) | 2.19 to 10.93% | 2.29 to 11.22% | Yes, qualified |
| 50 to 90% | Calcitic (calcareous) dolostone | “calcium carbonate makes up 10 to 50 % of a dolomite” (RR 99-03) | 10.93 to 19.67% | 11.22 to 19.77% | No |
| Over 90% | Dolostone | “composed dominantly of dolomite or ankerite” (RCS DOLO; “Formerly dolomite”) | Over 19.67% | Over 19.77% | No |
| 100% | Dolostone of pure dolomite | Mineral CaMg(CO3)2 (IMA) | 21.86% | 21.86% | No |
| Metamorphosed, dolomite dominant | Dolomitic metacarbonate-rock; marble as a field term | RR 99-02, sections 3.2 and 6.2.3 | — | — | No: metamorphic |
Is dolomite a limestone? What the BGS scheme says
No: it is a dolostone. The BGS Rock Classification Scheme entry for dolostone says sedimentary rocks “composed dominantly of dolomite or ankerite are given this group name”, and its short definition reads: sedimentary rock with components over 50% magnesium carbonates, over 50% of the magnesium carbonate being dolomite, “Formerly dolomite.” The limestone entry gives carbonate sedimentary rocks “in which the dominant carbonate mineral is calcium carbonate in the form of calcite, aragonite and/or vaterite.” The two are sister groups, not one inside the other.
Why people say dolomite for the rock. Because it used to be the rock’s name, as the BGS definition records. The dolomite sheet’s Name field says Dolomieu “contributed to early descriptions of the species in dolostone”. Today the IMA list uses dolomite for the mineral, CaMg(CO3)2, status G, 1792, Italy, and BGS uses dolostone for the rock. The same mineral-and-rock split is worked for lapis on is lapis lazuli a mineral or rock.
How dolostone forms, per the sheet. The Handbook dolomite sheet says dolomite forms “by diagenesis or hydrothermal metasomatism of limestone” and as a primary phase in hypersaline sedimentary environments. That is how a limestone can become a dolostone; this page gives no rate or depth, because the sheet gives none.
Where does limestone end and dolostone begin? The 50% line
The ladder. BGS Research Report RR 99-03 (sedimentary rocks), section 3, says dolomitic “is used as a qualifier where dolomite makes up 10 to 50 % of carbonate within a limestone”, and calcareous, “or more precisely a calcitic or aragonitic qualifier”, where calcium carbonate makes up 10 to 50% of a dolomite rock. Its section 3.2 defines the magnesium carbonate rocks as “dominantly (> 50%) composed of a magnesium carbonate”. Read together: under 10% dolomite, limestone; 10 to 50%, dolomitic limestone; 50 to 90%, calcitic dolostone; over 90%, dolostone.
The MgO lines, worked. On standard atomic weights CaMg(CO3)2 has a formula mass of 184.40 and is 21.86% MgO, 30.41% CaO and 47.73% CO2, as on the dolomite species page; calcite, Ca(CO3), is 56.03% CaO and 43.97% CO2 with no magnesium. In a rock of calcite and dolomite only, dolomite’s mass share of the carbonate is the rock’s MgO divided by 21.86. So the 10% line is 2.19% MgO, the 50% line 10.93% and the 90% line 19.67%. Substitute your own analysis: a rock reporting 15% MgO and no other magnesium mineral is 68.6% dolomite, a calcitic dolostone.
Documented failure: two assumptions. RR 99-03 does not say whether its percentages are by mass or volume. By volume, the lines move to 2.29%, 11.22% and 19.77% MgO, because dolomite at 2.86 is 5.53% denser than calcite at 2.7102. And the conversion fails if magnesium sits in another mineral: BGS gives magnesite-stones their own name, and magnesite is Mg(CO3). A bulk MgO figure alone cannot tell those cases apart.
Is dolomite a marble?
Not as a rock name in the BGS scheme. BGS Research Report RR 99-02 (metamorphic rocks) classifies metamorphic rocks with over 50% carbonate and calc-silicate minerals as calcsilicate- or metacarbonate-rocks; where the carbonate is dominantly dolomite, “the rock may be referred to as dolomitic metacarbonate-rock.” It says the name marble “has been widely used as well as misused”, including for some non-metamorphic carbonate rocks, “for example Purbeck Marble”, and for decorative stones such as the Portsoy Marble, which is a serpentinite.
Marble as a field term. Section 6.2.3 of the same report allows marble for rocks composed largely of calcsilicate and/or carbonate minerals where the proportions are unknown, to be reclassified after further study. So a white, sugary metamorphosed dolomite rock can be called marble in the field and dolomitic metacarbonate-rock once examined. The mineral itself occurs in such rocks: the dolomite sheet lists “a major component of some contact metamorphic rocks and marbles”.
What grows in heated dolomitic rock. Calc-silicate minerals named in the BGS metacarbonate definition include diopside, grossular, wollastonite and vesuvianite; on this site see diopside, grossular and wollastonite.
Is dolomite a mineral or a sedimentary rock?
Both words are in use, for different things. As a mineral, dolomite has its own row on the IMA list, CaMg(CO3)2, and its own Handbook sheet: hexagonal, hardness 3.5 to 4, density 2.86, perfect {10-11} cleavage. As a rock, the current BGS name is dolostone, a sedimentary rock. The general distinction between a mineral and a rock is on crystal vs mineral: what is the difference and, for quartz, is quartz a mineral.
Dolomite against calcite, from the sheets. Calcite, Ca(CO3), hardness 3, density 2.7102; dolomite, CaMg(CO3)2, hardness 3.5 to 4, density 2.86. A 20.00 g piece reads 12.62 g in water as calcite and 13.01 g as dolomite, a 0.39 g gap. Both cleave into rhombs, so cleavage does not separate them; the cleavage-then-density rule for the carbonates is on is aragonite the same as calcite, the three CaCO3 polymorphs on is aragonite a polymorph of calcite, and UV behaviour on is calcite UV reactive. This page gives no acid test.
Rock, not crystal. These numbers apply to single-mineral pieces. A rock of mixed calcite and dolomite is named by its proportions, which needs an analysis or a thin section; the in-water weight of a rock also depends on its pore space, which this page does not estimate. More guides are in the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.