138 Washington — Suggestion for Mineral Nomenclature. 



advance of, the classification and definitions of descriptive 

 chemistry, which only take cognizance of the ultimate chemi- 

 cal composition of the substance as revealed by analysis, and 

 its molecular structure as shown by its reactions, replacements, 

 molecular weight, etc. To the chemist, CaC0 3 is only calcium 

 carbonate, whether its crystal form is trigonal or orthorhombic, 

 with correlated differences in specific gravity, optic characters, 

 etc. To the mineralogist these two forms are different min- 

 erals, though the chemical composition is expressed by the 

 same empirical formula. 



This recognition of polymorphous or physically isomeric 

 forms of the same empirical molecule as different substances 

 and the converse relationship between substances of closely 

 similar crystal form and of the same chemical type, though dif- 

 fering in composition through isomorphous replacement, is 

 but a logical following out of the teachings of physical chem- 

 istry. It is an extension of the definition of "substance" to 

 include, in addition to the chemical composition, the relations 

 of the physical and chemical characters of matter to the con- 

 ditions of equilibrium which control crystallization. That the 

 recognition of the essential difference between substances (in 

 this sense) with the same ultimate chemical composition is 

 valid and necessary, is shown by consideration of such cases as 

 diamond and graphite (C), pyrite and marcasite (FeS 2 ), or cal- 

 cite and aragonite (CaC0 3 ). In these the differences in the 

 physical characters, and to no less an extent in many of the 

 chemical characters, as resistance to reagents, of the members 

 of each pair are so great that they must be regarded as dis- 

 tinct substances. This would be true even from the purely 

 chemical point of view, since the physical differences, espe- 

 ically those like specific gravity and specific heat, indicate 

 differences in the molecular weight and very probably in 

 molecular structure. 



It may be noted here that among minerals there are few 

 ascertained cases of chemical isomerism, in which the distinct 

 physical and chemical characters, due to profound and persist- 

 ent differences in the molecular structure, persist after the sub- 

 stance has been changed into an amorphous state, as by fusion 

 or solution, and then recrystallized.* Numerous illustrations 

 of this are furnished by organic chemistry, the most classical 

 being that of urea and ammonium isocyanate. Among 

 minerals it is difficult to prove the existence of such cases, 

 though they unquestionably occur, and Grothf regards pyrite 

 and marcasite, and cyanite, sillimanite and andalusite, respec- 



* Groth, Introduction to Chemical Crystallography, New York, 1906, p. 3. 

 f Groth, Chernische Krystallographie, Leipzig, Pt. I, 1906, p. 155. and 

 Pt. II, 1908, p. 258. 



