The present volume "Rare Earth Elements" A 6b deseribes in its first part the origin, mode of oeeurrenee, and behavior of Y and/or RE elements in the hydrosphere and atmos phere. Separately for marine and non-marine environments (surfaee, subsurfaee, mineral, and thermal waters), the behavior of RE (ineluding Y) in the hydrosphere eomprises espeeially the relationship between eontentleomposition and the ehemistry of water, and the processes aeting during migration, removal, and preeipitation are outlined; the influenee of biologieal material is mentioned. Behavior of RE in the atmosphere involves mainly transport, regional differenees, and temporal variations as weil as removal by preeipitation; the anthropogenie influenee is only outlined. The seeond part of this volume treats, partly in a more summary manner, the eosmo and geoehemieal eyeles and the balance of Y and/or RE elements. The relationship between geodynamie position and type of magmatism, as weil as the geoehemieal variations in the geospheres, especially mantle and erust of the earth, are deseribed in greater detail.

The volume "Rare Earth Elements" C 11 deals with the compounds and systems of the rare earth elements with boron, Le., borides, borates, and associated alkali double compounds. As in all earlier volumes of "Rare Earth Elements" Se ries C ("Seltenerdelemente" Reihe Cl, comparative data are presented in sections preceding treatment of the individual compounds and systems. Topics of the present volume C 11 aare the comparative data on the borides and the indi vidual sections on the systems and borides containing Sc, Y, and La. The individual sections for the systems and borides with Ce to Lu can be found in the following volume C 11 b together with the borates, their alkali double compounds, and other compounds containing Ce to Lu, boron and elements related by the Gmelin system. The most extensively studied borides treated in the comparative sections are of the type MB. These rare earth hexaborides are refractory compounds and so me of them, especially e LaBe, are good thermionic emissive materials or exhibit other interesting physical properties.

The volume "Rare Earth Elements" C10 deals with the rare earth tellurides, oxide tellurides, tellurates, telluride halides, tellurate halides, sulfide tellurides, selenide tellurides, and alkali rare earth tellurates. Another topic of this volume are the compounds of the rare earth elements with polonium. Sofaras meaningful and as in all earlier volumes of "Rare Earth Elements" Series C ("Seltenerdelemente" Reihe C), comparative data are presented in sections preceding treatment of the individual compounds and systems. Gaseous telluride molecules are described in the first section. The subsequent sections deal mainly with the chemical and physical properties of the solid tellurides, of which SmTe, EuTe, and TmTe play the most important rote. The most prominent feature of SmTe is the pressure-induced valence transition from divalent Sm to the intermediate valence state, similar to SmSe (see "Rare Earth Elements" C9). Main topics of the section on EuTe are the magnetic and spectroscopic investigations, which show only antiferromagnetic ordering at low temperatures, unlike the complex magnetic properties of EuSe. The magnetic phase diagram, magnetic resonances, exchange interactions, and anisotropies have been ex tensively studied. A pressure-induced valence change is also observed in TmTe with divalent Tm under ambient conditions. The valence changewas also reached by alloying with TmSe. The oxide tellurides have been studied less intensively than the other oxide chalcogenides and only the type M0 Te is known. Data on preparation, crystallographic, and magnetic 2 2 properties dominate this section.