LiAlSi2O6 III (high-quartz solid solution) was found to be hexagonal, space group P6222 (or P6422), a = 5.217 ± 0.001, c = 5.464 ± .002 Å, Z = 1, D o = 2.395 and D c = 2.399 g/cm3. The bond lengths were determined with an accuracy of 0.001 Å for Si,Al–O bonds and 0.005 Å for Li–O bonds. The final R value was 5.1%. The structure was confirmed to be a stuffed derivative of high quartz. The distribution of Si and Al in the tetrahedra is completely random. Lithium atoms are 4-coordinated and stuffed into interstitial positions parallel to the c axis. They occupy an equipoint of rank 3. There is only one lithium atom per unit cell, randomly distributed among the three equivalent sites. Each lithium tetrahedron shares two edges with two Si,Al tetrahedra. The Si,Al–Li distance, 2.609 ± 0.001 Å, is exceptionally short. An explanation for the unusually low thermal-expansion behavior of LiAlSi2O6 III is given.