CHARACTERISTICS OF SILICATES. 65 



CHARACTERISTIC BANDS OF QUARTZ AND OF SILICATES. 



About half the minerals known are silicates. Their percentage com- 

 position has been ascertained by quantitative analysis, but little has been 

 possible, as yet, in establishing their constitutional formula?. In organic 

 chemistry the constitution of compounds has been determined by vapor 

 density determinations, by the preparation of series of derivatives, by the 

 replacement of certain constituents by organic radicals, or by studying 

 their physical properties in solution. In mineralogy this has, as yet, not 

 been possible, and the constitution of many of the minerals has been 

 derived mainly from analogies with other substances which are better 

 understood. The silicates are sometimes grouped as follows: disilicates 

 (RSi 2 5 ) are salts of disilicic acid (H 2 Si 2 5 ), and have an oxygen ratio of 

 silicon to bases of 4: 1; polysilicates (R 2 Si 3 8 ) are salts of polysilicic acid 

 (H 4 Si 3 8 ), with oxygen ratio of 3:1; metasilicates (RSi0 3 ) are salts of 

 metasilicic acid (H 2 Si0 5 ), oxygen ratio of 2: 1; orthosilicates (R 2 Si0 4 ), salts 

 of orthosilicic acid (H 4 Si0 4 ), oxygen ratio of 1:1. 



Among the complex silicates, and often among the simple ones, not 

 only is the actual molecular structure in most cases doubtful, but even the 

 simple empirical composition of many species is still unsettled. A single 

 species may vary greatly in composition. This has been explained by 

 regarding the different forms as derivatives of a normal salt in which 

 various atoms or molecular groups may enter. It is not surprising then, 

 in the present limited study of their reflection spectra, to find no very 

 important relations among the reflection bands of silicates. 



From previous work it is difficult to decide what one ought to expect 

 in the question of the coincidence of bands in SiO, and in silicates. In 

 the case of the carbonates there are no bands (except a small depression 

 at 4.6 /j. in common with CO) coincident with those of C0 2 ; and we might 

 expect a similar condition to obtain in the silicates and Si0 2 . On the 

 other hand, in the carbohydrates having a structure indicating CH 2 and 

 CH 3 groups, there is a coincidence of certain characteristic bands with 

 those of ethylene (C 2 H 4 ) and ethane (C 2 H 6 ). The introduction of oxygen 

 atoms, however, modifies the spectrum, and, as a rule, there is no longer 

 a coincidence with the former bands. Furthermore, in S0 2 the maximum 

 at 8.7 11 is in coincidence with a similar band found in many sulphates, 

 while the band at 10.4 / is in common with a similar one in fuming sul- 

 phuric acid and is probably due to S0 4 . From this it is difficult to predict 

 what one ought to expect in the case of Si0 2 and of the silicates. From 

 previous observations that groups of chemically-related substances have 

 similar absorption spectra, one would expect to find a similar condition to 

 obtain in the various groups of silicates. The present investigation is not 

 extensive enough to draw general conclusions. Moreover, the constitution 

 of the minerals is in many cases in doubt, so that the lack of similarity in 



