It has been shown that provided the rock or mineral has been powdered to a sufficient fineness (e.g. 150–200 mesh) even the most refractory materials such as ilmenite, garnet, staurolite, and tourmaline may be opened up by the perchloric-hydrofluoric acid attack. It has also been shown that if barium and/or strontium are present they can be removed by simple addition of sulphuric acid and filtration, since calcium is entirely unaffected by this procedure. Finally, it has been shown that, if desired, the calcium may be extracted as oxalate in acetic acid solution without precipitation of the Fe group, and without contamination by this group.