Editing Mineral (section)

====Carbonates====
{{main article|Carbonate minerals}}
The [[carbonate minerals]] are those in which the main anionic group is carbonate, [CO<sub>3</sub>]<sup>2−</sup>. Carbonates tend to be brittle, many have rhombohedral cleavage, and all react with acid.<ref>{{harvnb|Chesterman|Lowe|2008}}, p. 431</ref> Due to the last characteristic, field geologists often carry dilute hydrochloric acid to distinguish carbonates from non-carbonates. The reaction of acid with carbonates, most commonly found as the polymorph calcite and [[aragonite]] (CaCO<sub>3</sub>), relates to the dissolution and precipitation of the mineral, which is a key in the formation of limestone caves, features within them such as stalactite and stalagmites, and [[karst]] landforms. Carbonates are most often formed as biogenic or chemical sediments in marine environments. The carbonate group is structurally a triangle, where a central C<sup>4+</sup> cation is surrounded by three O<sup>2−</sup> anions; different groups of minerals form from different arrangements of these triangles.<ref>{{harvnb|Dyar|Gunter|2008}}, p. 667</ref> The most common carbonate mineral is calcite, which is the primary constituent of sedimentary limestone and metamorphic marble. Calcite, CaCO<sub>3</sub>, can have a significant percentage of magnesium substituting for calcium. Under high-Mg conditions, its polymorph aragonite will form instead; the marine geochemistry in this regard can be described as an [[aragonite sea|aragonite]] or [[calcite sea]], depending on which mineral preferentially forms. [[Dolomite (mineral)|Dolomite]] is a double carbonate, with the formula CaMg(CO<sub>3</sub>)<sub>2</sub>. Secondary dolomitization of limestone is common, in which calcite or aragonite are converted to dolomite; this reaction increases pore space (the unit cell volume of dolomite is 88% that of calcite), which can create a reservoir for oil and gas. These two mineral species are members of eponymous mineral groups: the calcite group includes carbonates with the general formula XCO<sub>3</sub>, and the dolomite group constitutes minerals with the general formula XY(CO<sub>3</sub>)<sub>2</sub>.<ref>{{harvnb|Dyar|Gunter|2008}}, pp. 668–69</ref>