metacarbonate

Metacarbonate is a term used in metamorphic petrology to describe carbonate-rich rocks that have been transformed by metamorphism. It encompasses rocks derived from carbonate protoliths such as limestone and dolostone, through regional or contact metamorphic processes. When calcite dominates, the rock is commonly called marble; when dolomite is prevalent, it is a dolomitic marble. More broadly, metacarbonates can include calcsilicate rocks that form during metamorphism and contain both carbonate minerals and silicates.

Formation and textures

Metacarbonates form under elevated temperature and pressure conditions that promote recrystallization and grain growth of carbonate minerals. They are typically non‑foliated to weakly foliated; with increasing metamorphic grade, textures evolve from a mosaic of interlocking calcite or dolomite grains to more elongated or stretched grains along shear directions. Fluid activity during metamorphism can introduce accessory minerals such as diopside, tremolite, scapolite, or wollastonite, especially in calcsilicate assemblages.

Mineralogy and identification

The dominant minerals are calcite and/or dolomite. Accessory minerals vary with metamorphic grade and fluid composition. In many marbles, quartz is scarce, but it may be present in trace amounts or within calcsilicate zones. Chemically, metacarbonates are reactive to dilute acids, displaying effervescence characteristic of carbonate minerals. Their appearance ranges from light, uniform colors to banded or veined textures depending on original depositional features and metamorphic history.

Occurrence and significance

Metacarbonates are widespread in ancient orogenic belts and other tectonically active regions where carbonate sedimentary rocks have undergone metamorphism. Economically, marble and related metacarbonates have long been quarried for decorative stone and industrial lime production, while their metamorphic textures record the tectonothermal history of their terranes.