The sulfate carbonates are a compound carbonates, or mixed anion compounds that contain sulfate and carbonate ions. Sulfate carbonate minerals are in the 7.DG and 5.BF Nickel-Strunz groupings.
They may be formed by crystallization from a water solution, or by melting a carbonate and sulfate together.
In some structures carbonate and sulfate can substitute for each other. For example a range from 1.4 to 2.2 Na<sub>2</sub>SO<sub>4</sub>â¢Na<sub>2</sub>CO<sub>3</sub> is stable as a solid solution. Silvialite can substitute about half its sulfate with carbonate and the high temperature hexagonal form of sodium sulfate (I) Na<sub>2</sub>SO<sub>4</sub> can substitute unlimited proportions of carbonate instead of sulfate.