Carbonate Minerals
Carbonate minerals are the rock-forming constituents of limestones and dolomites. The principal carbonate minerals fall into three crystallographic groups — the calcite group (rhombohedral), the dolomite group (rhombohedral), and the aragonite group (orthorhombic) — and only a few of the minerals within these groups are important components of sedimentary rocks. [1] The distinction between these groups matters practically because the two common CaCO3 minerals — calcite and aragonite — differ not just in crystal structure but in stability, with aragonite being the metastable form that converts to calcite over geologic time.
The Calcite Group
All members of the calcite group crystallize in the rhombohedral system.
Calcite (CaCO3) is the dominant mineral of limestones, especially in rocks older than the Tertiary. [1] Its dominance in ancient limestones reflects the diagenetic conversion of the originally more common aragonite to the thermodynamically stable calcite form.
Magnesite (MgCO3) is uncommon in sedimentary rocks but occurs in some evaporite deposits. [1]
Rhodochrosite (MnCO3) is uncommon in sedimentary rocks but may occur in Mn-rich sediments associated with siderite and Fe-silicates. [1]
Siderite (FeCO3) occurs as cements and concretions in shales and sandstones, is common in ironstone deposits, and also occurs in carbonate rocks altered by Fe-bearing solutions. [1]
Smithsonite (ZnCO3) is uncommon in sedimentary rocks but occurs in association with Zn ores in limestones. [1]
The Dolomite Group
Dolomite (CaMg(CO3)2) is the dominant mineral in dolostones and is commonly associated with calcite or evaporite minerals. [1] Unlike high-magnesian calcite, which merely has Mg randomly substituting for Ca in the calcite crystal lattice, true (stoichiometric) dolomite is a totally different mineral in which Mg ions occupy exactly half of the cation sites and are arranged in well-ordered planes that alternate with planes of CO3 ions and Ca ions. [1] This ordered structure is what makes dolomite difficult to precipitate at surface temperatures, a problem discussed in detail on the Dolomite Problem page.
Dolomite occurs in a few restricted modern environments, particularly in certain supratidal environments and freshwater lakes, but it is much less abundant in modern carbonate environments than aragonite and calcite. [1]
Ankerite (Ca(Mg,Fe,Mn)(CO3)2) is much less common than dolomite and occurs in Fe-rich sediments as disseminated grains or concretions. [1]
The Aragonite Group
All members of the aragonite group crystallize in the orthorhombic system, in contrast to the rhombohedral symmetry of the calcite group. This structural difference — not chemical composition — is what distinguishes aragonite from calcite.
Aragonite (CaCO3) is a common mineral in recent carbonate sediments but alters readily to calcite. [1] It is the dominant carbonate mineral secreted by a wide range of modern marine organisms, including molluscs, scleractinian corals, and calcareous green algae. [1] Its metastability means it is rare in ancient rocks; virtually all of the aragonite originally deposited before the Cretaceous has been diagenetically replaced.
Cerussite (PbCO3) occurs in supergene lead ores. [1]
Strontianite (SrCO3) occurs in veins in some limestones. [1]
Witherite (BaCO3) occurs in veins associated with galena ore. [1]
Other carbonate minerals such as magnesite, ankerite, and siderite are even less common than dolomite in modern sediments. [1]
Related Topics
Crystal
The word crystal is used in two distinct ways in mineralogy, and the difference between them matters more than it might first appear.{/* SRC: Nesse p.5: "grains. It might be better to use the term...
Crystal System
Every mineral belongs to one of six crystal systems, defined by the geometry of its unit cell - specifically the lengths of the three cell edges (a, b, c) and the angles between them (α, β, γ).{/*...
The Dolomite Problem
Dolomites are calcium carbonate rocks composed of more than 50 percent of the mineral dolomite [CaMg(CO3)2], and to avoid confusion between the rock and the mineral they are...
Mineral
"Mineral" means different things depending on who you ask.{/* SRC: Nesse p.3: "Almost every human endeavor is influenced by minerals. Many natural resources used in the manufacture of" */}...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs
Master UPSC Geology Optional
Ex-ONGC Geologist & Rank Holder
Learn the exact analytical answer-writing patterns needed for UPSC Optional from an AIR 2 & AIR 25 holder.
Offline in Delhi