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AuszugEs werden die Merkmale der beobachteten Strukturphasen der Plagioklasreihen beschrieben zusammen mit einer Übersicht über die wahrscheinlichen Zustandsbereiche und über die Art der Strukturumwandlungen. Abb. 3 zeigt das für jeden Strukturtyp charakteristische reziproke Gitter, Abb. 4 ist ein Diagramm der Zustandsbereiche, in denen diese Typen auftreten. Über alle Übergänge zwischen mehr oder weniger gut definierten Strukturformen hinweg bleibt das Silikatgerüst der Plagioklase im wesentlichen unverändert. Ein allgemeines Schema der Umwandlungen wird sich jedoch erst aufstellen lassen, wenn die wichtigsten Strukturtypen hinreichend genau bestimmt sein werden. AbstractThe characteristics of the various structural phases recognised over the plagioclase series are described, together with a general account of their probable composition fields and the manner in which the transitions between structure types take place. Fig. 3 shows the form of the reciprocal lattices characteristic of each structure type, whilst Fig. 4 shows diagrammatically their occurrence. Throughout the transitions between the variety of forms more or less welldefined in character, the essential felspar silicate framework remains unchanged. The general scheme of structural changes can only be resolved when the key structures have been determined with precision.Z. Krlstallogi. Bd. Ill, 1 1Brought to you by | University of California Authenticated Download Date | 6/13/15 12:55 AM

SummaryAn 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.