Carbonate rocks predominantly consist of carbonate minerals, primarily calcite (CaCO3) and dolomite (CaMg(CO3)2). Other carbonates, like aragonite (another form of CaCO3), siderite (FeCO3), and ankerite (Ca(Fe, Mg)(CO3)2), contribute but in smaller amounts. The proportion of these minerals dictates the rock’s physical properties and classification.
Formation Processes
These rocks form through diverse processes. Biogenic formation dominates, with organisms like corals, mollusks, and foraminifera secreting calcium carbonate shells and skeletons. Accumulation of these bioclasts creates limestone. Inorganic precipitation also plays a role, particularly in high-saturation environments like evaporating lagoons or around hydrothermal vents. This precipitation generates oolitic or micritic limestones.
Classifying Carbonate Rocks
Classification often relies on grain size, composition, and texture. Limestone, the most common type, ranges from coarse-grained (composed of visible fossils and grains) to fine-grained (micritic, with grains too small to see without magnification). Dolomite, another significant type, typically forms through the alteration of limestone by magnesium-rich fluids. The table below summarizes key characteristics:
| Limestone | Calcite | Biogenic, inorganic precipitation | Clastic, micritic, oolitic |
| Dolomite | Dolomite | Replacement of limestone | Crystalline |
| Chalk | Calcite | Accumulation of microscopic organisms | Fine-grained, porous |
Factors Influencing Rock Properties
Several factors affect the final rock characteristics. Water chemistry influences mineral precipitation. Burial depth and temperature affect diagenesis (post-depositional changes) altering texture and mineralogy. Sedimentary environment significantly influences grain size and sorting.
