Introduction

Metamorphism of a pure calcite limestone can produce a beautiful white calcite marble that is great for statues but does not provide much information about its metamorphic P-T path. However, metamorphosed dolomite with some quartz sand mixed in may have a variety of Ca-Mg-Si minerals that offer evidence regarding the conditions of metamorphism in a chemically tractable system. H₂O must be added to produce some of the minerals and CO₂ is released in carbonate-consuming reactions.

Mineralogy of Metamorphosed Carbonate Silicate Rocks

Calcite and dolomite are the carbonate minerals most likely to occur in metamorphosed carbonate silicate rocks. Calite and dolomite will look very similar in thin section, so some petrographers stain the calcite in their carbonate-bearing thin sections with Alizerin Red S. If the protolith is dolomitic limestone with quartz sand, the metamorphic silicate minerals that most commonly occur are talc, tremolite, diopside, forsterite, and wollastonite. If no Fe is present, these minerals are all colorless. A small amount of iron will add some green color to these minerals in hand sample, although more iron is needed to make them colored in thin section than in hand sample. If some mud is also in the protolith, Al-bearing minerals may be present such as phlogopite, clinzoisite, and plagioclase. A list of minerals commonly observed in metamorphosed carbonate silicate rocks and links to the properties of these minerals can be found here.

Samples of Metamorphosed Carbonate Silicate Rocks

The Rock Library includes images of hand samples and thin sections of metamorphosed carbonate silicate rocks. The IUGS recommends that rocks >95% carbonate minerals be called pure marbles, rocks with 50% to 95% carbonate minerals be called impure marbles, rocks with 5% to 50% carbonate minerals be called carbonate-silicate rocks, and those with <5% carbonate be called calc-silicate rocks. Links to images of metamorphosed carbonate silicate rocks can be found on this list.

SCM Diagram

The mineral assemblages and chemical reactions of metamorphosed siliceous dolomite rocks can be represented nicely on an SiO2-CaO-MgO (SCM) diagram. This diagram assumes that the volatiles H₂O and CO₂ are free to come and go and is a projection from those oxides. You can plot bulk rock compositions from your own chemical data (upload a .csv file) on an SCM diagram by clicking here. By clicking buttons on the linked SCM diagram you can see where the compositions of common minerals plot on the diagram.

Chemical Reactions in Metamorphosed Carbonate Silicate Rocks

Chemical reactions among carbonate and silicate minerals may consume from or yield to the metamorphic fluid H₂O and/or CO₂. This means that equilibrium mineral assemblages of metamorphosed carbonate silicate rocks depend on the mole fraction X_CO2 [= CO₂/(CO₂ + H₂O)] in the fluid as well as on temperature, pressure, and bulk composition. You can see this by clicking on the T-X_CO2 diagram on the right. When the diagram opens, you will see unvariant reaction curves. Click on the "Show Assemblages" button and mouse over the T-X_CO2 graph to see the mineral assmblages that are stable between reactions. Then click on the "Change Pressure" button and use the slider to see how the reactions change with pressure.

Metamorphosed Shale