A Natural Modification of Polymorph Na ₂ SO ₄ (III) in Halite‐Hosted Sulfate‐Rich Melt Inclusions From the Udachnaya‐East Kimberlite

Anhydrous Na 2 SO 4 , known by phase transitions on heating, has been reported mostly as a low temperature structural form, mineral thenardite in widely diversified geological environments. Recently, coexisting high temperature Na 2 SO 4 polymorphs were found in volcanic sublimates. Here we present the finding of polymorph Na 2 SO 4 (III) in sulfate‐rich melt inclusions from chloride enclaves of the Udachnaya‐East kimberlite (Siberia). A natural analog of synthetic metastable Na 2 SO 4 (III) is documented for the first time in melt inclusions. Phase III was stabilized by the incorporation of Ca‐ions, based on SEM‐EDS. Raman imaging highlighted the regular distribution of two Na 2 SO 4 polymorphs in melt inclusions. Na 2 SO 4 (III) was found in a Ca‐rich solid solution, in contrast to Na 2 SO 4 (V), located in a Ca‐depleted sulfate mixture. Identifications of polymorphs in melt inclusions is of special importance, as solidified former liquid inclusions provide the only means to directly establish the physio‐chemical conditions of paleo fluid.