Summary: An X-ray study of a number of rare-earth silicates has been carried out; single-crystal and powder data are presented.

It is found that cerites can contain up to at least 6 % CaO without disruption of the structure, which is trigonal with probable space-group P321 etc. The cell dimensions are a ca. 10·8 Å., c ca. 37–38 Å., although there is a strong pseudo-cell with halved c-axis. An ideal formula (Ca,Ln†)₃Si₂(O,OH,F)₉ is proposed for this series.

Lessingite and beckelite can be regarded chemically as lime-rich members of this cerite series. They do not, however, have the same crystal structure as cerite; their diffraction patterns are very similar to that of britholite, and indicate a structure dimensionally comparable with apatite. For a hexagonal cell, the dimensions are a ca. 9·7 Å., c ca. 7·1 Å., with probable space-group P6₃, &c. An ideal formula (Ca,Ln)₂(Si,Al,P)(O,OH,F)₅ is proposed for this series. Marked biaxial optical properties suggest that the structure may be truly orthorhombic, with a very close dimensional approximation to a hexagonal cell.

The new mineral, stillwellite, is unrelated to either of these series. It is trigonal with cell dimensions a ca. 6·9 Å., c ca. 6·7 Å., with probable space-group P3₁12. An ideal formula (Ca,Ln)(Si,Al,P)B(O,OH,F)₅ suggested by previous work is confirmed.