The only metal which has an abundance in the universe above 0.1% is iron, so that's going to be the cation of the mineral. Oxygen is a preferred reaction partner of iron and it's the third most abundant element in the universe, so iron oxide is the hottest candidate here IMO.

It should be mentioned that iron hydrides have been observed in space:

Article Iron hydrides formation in interstellar clouds

I haven't found claims that these are highly abundant, though.

OK, nice idea. However, it should be considered the possibility, that the carbon produced by stars is the third-fourth most abundant element in the universe just after H, He and O(?) and there are probably white/brown dwarfs composed mostly/ entirely of carbon. This is the mass which cannot be ignored.

Yes, but a star is in plasma state, so none of the carbon in those stars is mineralic.

Of course, those which are still active, but the star life ends as black dwarfs which are cooled objects. Approximately 90 percent of all stars end their lives as white dwarfs and dim slowly. That type will be more abundant as the universe will be older and older.

Ok, the state of the solid matter should be clarified in so dense objects as dwarfs are. It is probably a solid matter (degenerate matter), which I do not know if it can be considered to form crystalline structure. It is probably a rigid and densely packed lattice of atomic nuclei. Perhaps, you right and it is not diamond, but the dense solid phase composed of carbon.

Olivine and its polymorphs, wadsleyite and ringwoodite, in large bodies, this decomposes into bridgmanite + Fe-periclase. In even larger bodies, probably this Mg-Si PEROVSKITE decomposes into oxides such as MgO, FeO and SiO2. Regards, Sebastian.

Dear All: substances present in stars, nebulae, and gas clouds cannot be called minerals, as minerals have to be solid and have an ordered internal structure (crystalline structure). Therefore, minerals sensu stricto can exist ONLY in rocky planets, or the rocky cores of gaseous planets, or the rocky cores of their satellites. Therefore, as meteorites clearly show, the most abundant mineral is olivine, (Mg,Fe)2SiO4, present in chondrites, and the voluminous rocky mantles or cores of planets or satellites, but it goes through polymorphic transitions, and even decomposition reactions in large bodies such as Earth, and superearths. Some experiments have shown that, at extremely high pressures, such as those in the lower mantle of large rocky superaearths, all minerals present will be oxides such as ferrite Feo, stishovite SiO2 and periclase MgO, so probably periclase MgO is the most abundant mineral in the universe, being Mg more abundant than Fe in a cosmic scale. Regards. Sebastian.

Dear @Sebastián Grande

and what about extinct stars (cooled carbon-rich objects). The Universe includes more solid phase than that of terrestrial objects.