170 A. L. DAY MINERAL RELATIONS FROM LABORATORY VIEWPOINT 



the physical properties change continuously with the composition. Iso- 

 morphous mixtures are most commonly found between minerals of closely 

 related atomic or molecular grouping — that is, of similar chemical compo- 

 sition or crystal habit. The concentration may vary slightly or consider- 

 ably in the same hand specimen, depending on temperature or other 

 changes during its crystallization. Its distinctive feature, which is famil- 

 iar both to the field and to the laboratory student, is the continuity of the 

 changes of its physical properties with the changes in percentage compo- 

 sition of its ingredients. It may be added, for the sake of clearing up a 

 possible misunderstanding, that the term "solid solution,'^ as defined by 

 the physical chemists, applies to crystalline solutions of this kind, and 

 not to the glasses, as is sometimes stated. The glasses from this view- 

 point are still ultra-viscous liquids, and retain the "latent heat of fusion" 

 which would have been given off had a change of state occurred. 



The eutectic is a mixture in which the component minerals bear a defi- 

 nite relation to each other, but neither loses its identity. It must not be 

 regarded as in any way analogous to a compound. It has been commonly 

 supposed that the simultaneous crystallization of two minerals in eutectic 

 relation always resulted in a peculiar close-grained interweaving of the 

 crystals to which the name "eutectic structure" has been applied. Such a 

 "eutectic structure" is usual in metallic alloys and is readily identified, 

 Init pure minerals cooling under laboratory conditions do not show it. 

 Whether the peculiar interweaving observed in graphic granite is a true 

 eutectic structure of quartz and feldspar, or merely another of the effects 

 of an earlier intermixture of certain volatile ingredients no longer found 

 there, can not receive a positive answer. Such laboratory experience as 

 we have appears to indicate that this structure is the result of a much 

 more complicated activity than the mere cooling of quartz and feldspar in 

 eutectic ( ?) proportions. The appearance of a eutectic in a solution of 

 two minerals can be recognized in a variety of ways, but it is extremely 

 doubtful if any structure has yet been found to be characteristic of the 

 eutectic relation between minerals. 



The appearance of a compound in a series of mineral mixtures breaks 

 up the series abruptly into two, either of which may possess any of the 

 characters above outlined. These may be treated quite independently of 

 each other for purposes of systematic study of any kind. The compound 

 possesses independent physical and chemical properties which can be 

 determined for the purpose of distinguishing it from all other com- 

 pounds, provided it can be separated out and independently studied. The 

 mere appearance of particular optical properties in the grains of a rock 

 section, for example, while it may suffice for the immediate identification 

 of a known mineral, is not sufficient for the establishment of a new one. 



