Dolomite (dolostone)

The acid test, done properly

Standard practice is two tests, not one. First put a drop of 10% hydrochloric acid on a fresh surface. On limestone the reaction is immediate and loud; on dolostone there is either nothing visible or a few slow, sparse bubbles that take ten seconds or more to appear. Then scratch a little powder onto a clean surface and put acid on the powder. Dolomite powder fizzes readily because the reaction rate depends on surface area, and the powder removes that obstacle. A rock that fails the first test and passes the second is dolostone.

Naming: mineral versus rock

There is a naming convention worth getting right, because published descriptions follow it inconsistently. Dolomite is the mineral, CaMg(CO₃)₂ (per Geology.com Mineral Encyclopedia). Dolostone is the rock made mostly of it. Many papers and most field guides use "dolomite" for both, which creates real confusion when a rock is described as "dolomitic limestone" — that phrase means a limestone with 10 to 50 percent dolomite, and it is a distinct and very common category in its own right.

Where the magnesium comes from

This is the part geologists still argue about. Dolomite is rare in modern sediments and abundant in ancient ones, which is the "dolomite problem": seawater is supersaturated with respect to dolomite, yet it barely precipitates today. The working explanation is that most dolostone is secondary — limestone that was altered after deposition by magnesium-bearing fluids, either seawater concentrated by evaporation on a tidal flat, or brines circulating through the rock during deep burial. The field evidence for this is that dolostone bodies usually cut across the original bedding, replacing limestone along particular horizons or along fracture networks rather than forming neat primary layers.

Reading dolostone in the field

Field recognition goes beyond the acid bottle.

• Weathering texture: dolostone develops a distinctive sugary or sandy surface as the softer parts dissolve and the rhombs stand proud.
• Colour: often a warmer buff or cream than the grey of an adjacent limestone, and frequently with a yellowish or orange weathering crust.
• Hardness: a copper coin scratches limestone easily and dolostone only with pressure; a knife scratches both but grooves deeper in limestone.
• Associated minerals: look for small white saddle-shaped rhombs, and for galena, sphalerite or pyrite veins in the Mississippi-Valley-type districts.

Practical consequences

The slow dissolution rate has two visible outcomes. Dolostone makes better aggregate and dimension stone than limestone, and it forms resistant caprocks — the Niagara Escarpment exists because a Silurian dolostone layer outlasts everything around it. In karst regions, dolostone produces a different landform suite from limestone: fewer caves and towers, more broad pavements and rounded ridges, because the dissolution is slower and less selective. If you are mapping a carbonate sequence, the limestone-dolostone boundary is usually also the boundary between two different styles of landscape.

Sources

Hardness, streak, cleavage and specific-gravity values quoted on this sheet are checked against the references below.

• Geology.com Mineral Encyclopedia — mineral property database — hardness, cleavage, streak, specific gravity and chemical composition per mineral
• Geology.com Rocks — rock-type reference — descriptions and photographs of igneous, sedimentary and metamorphic rocks
• National Park Service — U.S. National Park Service geology pages — mineral definition, physical properties and common minerals