Oxalate sulfate
Oxalate sulfates are mixed anion compounds containing oxalate and sulfate. They are mostly transparent, and any colour comes from the cations. Related compounds include the sulfite oxalates and oxalate selenates. Production 
Oxalate sulfates may be deposited from a solution in water of the metal anions and sulfate with oxalic acid when evaporated. Crystals formed this way may be hydrates. Properties 
Many crystal forms are non-centrosymmetric and have non-linear optical properties. When heated, oxalate sulfates will first dehydrate, and then give off carbon dioxide. List

References

Oxalates
Mixed anion compounds
Sulfates