Recently Fuchs, et al., (1966) described a new mineral, roedderite, from the Indarch (enstatite chondrite) meteorite. Roedderite is essentially (K2O, Na2O):5MgO: 12SiO2, or (K,Na)2Mg5Si12O30. This was the first natural occurrence of a compound which Roedder (1951) encountered in his study of the synthetic system K2O-MgO-SiO2. Fuchs, et al., presented six microprobe analyses of which one was arbitrarily chosen for comparison here (Table 2). In addition, synthetic K2O:5MgO:12SiO2, which was kindly supplied by Dr. E. Roedder, was analyzed by microprobe and the results are presented also. Prior to this the 1:5:12 proportions for this synthetic material were not the result of direct analysis but were based on a careful geometrical analysis of the phase diagram for this system (Roedder, 1951). This was the only method possible for microscopic grains prior to the ready availability of the electron microprobe.