A magnesium carbonate mineral. It is quarried in California, Washington, Austria, Russia, Manchuria, Greece and Slovakia and processed from sea water. Sources are reasonably consistent in the CO2 content, but can vary widely in the iron and significantly in the MgO (the analysis given here is theoretical).
Some material data sheets show both the oxide and mineralogical analyses. Dolomite, for example, is composed of calcium carbonate and magnesium carbonate minerals, these can be separated mechanically. Although this material participates in the glaze melt to source the MgO and CaO (which are oxides), it's mineralogy (the calcium and magnesium carbonates) specifically accounts for the unique way it decomposes and melts.
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Magnesite at Wikipedia