A new occurrence of kornerupine in the skarn zone at Mautia Hill, Tanganyika, has yielded unit-cell dimensions a 16·100, b 13·767, c 6·735 Å, density 3·29₇, and composition Na0·01Mn0·01Ti0·02Mg3·69Fe0·37IIIAl6·36Si3·71B0·41O21·75(OH)0·25. The unit-cell clearly contains 88 anions rather than the 86 proposed previously. Grandidierite from Sakatelo, Madagasear, has unit-cell dimensions a 10·978, b 10·335, _c 5·760 Å, space group Pna2₁ or Pnam, density 2·97₆, and composition Fe0·12IITi0·01Mg0·88Fe0·03IIIAl3·00Si1·00B0·97O9·00. The unit-cell contains 36 anions and the structure appears to involve A10₆ chains parallel to the c-axis.