DOLOSTONE
Dolostone (formerly “dolomite”) is a chemical sedimentary rock
composed of the mineral dolomite (CaMg(CO3)2 -
calcium magnesium carbonate). Dolostone can be jokingly described as the “ultimate
non-descript rock”. It often looks like driveway gravel.
Well, there’s a reason for this - a lot of driveway gravel is dolostone
(at least around where I live).
Most dolostones are grayish,
microcrystalline-textured, and have vuggy porosity. Vugs are irregularly-sized &
shaped cavities. Some dolostones
are fossiliferous. Fossiliferous dolostones usually have poorly-preserved
fossil “ghosts” (see below). Rarely, fine-grained dolostones
have soft-bodied fossil preservation.
Crystalline-textured dolostones appear secondary in
origin. They are typically
interpreted as chemically-altered fossiliferous limestones. Some
dolostones look primary, but how they formed is not entirely
clear. Chemically, all that's needed to form dolostone is the addition of
magnesium (Mg) to limestone. The details of this chemical change are not
fully understood. A few localities on Earth do have dolomite or
protodolomite forming now, but the details of the dolomite-forming process are still significant unsolved problems in
sedimentary geology - “the dolomite problem”.
Dolostone of Silurian age with vuggy porosity (= the irregularly-sized &
shaped holes) and fossil “ghosts” (the somewhat rounded, empty
structure on the right side is an external mold of a fossil brachiopod).