Riffdolomite

Reef dolomite is a carbonate rock consisting mainly of the mineral dolomite. Dolomite has the chemical formula CaMg(CO3)² and occurs in the rock reef dolomite to at least 90 percent. If the dolomite content is lower, it is a dolomitic limestone. Reef dolomite is rarely white, but often occurs in ivory, light gray, grayish yellow or greenish gray shades.

Compared to the chemically related limestone, reef dolomite is somewhat harder and much more brittle. On fresh fractured surfaces, dolomite stones are often gray, but usually weather to a yellowish or brownish color. A characteristic feature of dolomite rock is the splintery rock surface as opposed to the smooth washed appearance of limestone. In addition, dolomite reacts slowly with cold dilute hydrochloric acid compared to limestone, which allows them to be distinguished from each other. While the reaction of hydrochloric acid with limestone produces vigorous carbon dioxide bubbles, hardly any bubbles develop in dolomite.

Dolomite rocks can be formed either by primary precipitation of dolomite or by secondary dolomitization of limestone mud. Dolomite rocks can develop into solid sedimentary rocks by diagenesis, the process of compaction of sediments. This process involves the deposition of sediments that gradually become thicker layers and are eventually compacted into sedimentary rock under the pressure of their own weight.

Mechanical properties of reef dolomite can vary, but they are typically observed within a certain range. Flexural tensile strength ranges from 9 to 15 N/mm², compressive strength ranges from 125 to 250 N/mm², and modulus of elasticity ranges from 16,000 to 80,000 N/mm². The bulk density of reef dolomite varies between 2400 and 2900 kg/m³.

Reef dolomite rock is often praised for its sugar-grained structure, which is due to the structure of fine crystals. These crystals are finer grained than sugar, but coarser grained than limestone crystals. In sunlight, the cleavage surfaces of reef dolomite can visibly glitter. The fine crystallinity of the rock results from recrystallization during or shortly after sedimentation. However, the stratigraphic assignment of dolomitic rock formations can be complicated by recrystallization, since structural features and fossils of the source rock are blurred.

Location of the rock reef dolomite in Orlawell

Image: Thomas Voigt