33 14. 3. 5 REE between Plagioclase and Aqueous Fluid 0 Cullers et al. (1973) measured the distribution of REE at 850 C and 750 bars pressure between a natural plagioclase, An, and gaseous water. The rare earths 65 favored the plagioclase by a factor which varies from about 25 for Ce to 10 for Lu. Data were also obtained for forsterite, diopside, enstatite and two rhyolite glasses, on the one hand, and water on the other hand, thereby permitting estimation of the partition coefficients between all pairs of phases. 14. 4 Chemical Substitution in Natural Feldspars 14. 4. 1 Introduction It is quite impracticable to give all the data on chemical substitution in natural feldspars: indeed many of the details are significant only to some particular pegmatite or rock body. As far as possible, emphasis is placed on features of general interest to crystal chemists and to petrologists. Ironically the well established features can be described more easily than the uncertain ones, and unfortunately it is necessary to use valuable space on data of dubious value. The bibliography is fairly complete, but it was impracticable to locate all data, especially those in obscure journals. Each reference is followed by a list of the elements referred to in the paper, thereby permitting a reader to compile a fairly compre hensive set of references on any chosen element. Not all papers are mentioned in the text. The book on Geochemistry and Mineralogy of Rare Elements, etc.