Magnesiadolomita

Magnesiadolomita, also known as magnesian dolomite, is a magnesium-rich variety of the carbonate mineral dolomite. Its composition lies along a solid solution between calcium dolomite and magnesium dolomite, commonly expressed as (Ca,Mg)(CO3)2, with higher magnesium content identifying it as magnesian.

Magnesian dolomite shares the same crystal structure as dolomite, a trigonal-rhombohedral carbonate with alternating layers of calcium and magnesium. In hand specimen and thin section it resembles dolomite, typically presenting a white to gray color that may be influenced by impurities such as iron, manganese, or trace elements. It is relatively soft, with a hardness around 3.5–4 on the Mohs scale, and a specific gravity in the vicinity of 2.8–2.95. Like dolomite, it generally shows rhombohedral cleavage and faint effervescence in dilute acids.

Formation and occurrence: magnesian dolomite forms through diagenetic processes in which magnesium-rich fluids replace or alter preexisting limestone or early dolostone, increasing the Mg content of the mineral. It is especially common in carbonate platforms and basinal settings where magnesium-rich fluids circulate during burial or tectonic events. Magnesian dolomite-bearing rocks include dolostones and partially altered limestones and can serve as important reservoir rocks for fluids in some basins.

Significance: as a compositional variety of dolomite, magnesian dolomite aids geologists in interpreting diagenetic histories, fluid sources, and the evolution of carbonate rocks. It is distinguished from calcite- or magnesium-poor dolomite by its enhanced magnesium content and related textural and geochemical signatures.