34 CLASSIFICATION BASED ON MINEEAL COMPOSITION. 



combinations of either (1) anorthite and labradorite. or (2) anorthite and 

 albite (orthoclase), or (3) labradorite and albite (orthoclase). 



In 1850 Delesse said : " I have ah-eady had occasion to remark that we 

 have hitherto attached too much importance to the varieties of the feldsjDars 

 of the sixth crystalHne system, and that nature has not always been limited 

 by the divisions established among them by chemists and geologists ; the 

 same rock sometimes containing several varieties of these feldspars." * In 

 the last reference Delesse had also pointed out that the composition of the 

 feldspar was not constant even in the same rock from the same locality. 



In 1851 Hermann taught that in the feldspars were two molecules : one 

 denoted by a = (R Sig -|- Sig), 



and the other by b = (R Si -j- Si). 



Of these molecules the species were thus compounded (giving, however, 

 from his list only those species of importance to-day) : — 



Orthoclase = a 



Albite = a 



Anorthite =^ b 



Andesite = '' "^  



Labradorite = " *"' 



•i 



Oliffoclase = iil_± — ? 



The union in this case was regarded as a molecular union, and not a 

 chemical combination between the atoms, t 



Sartorius von Waltershausen, in his work " Ueber die vulkanischen 

 Gesteine in Sicilien und Island, und ihre submarine Umbildung," published 

 in 1853, advanced the theory that there were three definite triclinic 

 feldspars, i. e. anorthite, albite, and krablite, and that the other triclinic feld- 

 spars were formed by varying compounds of these. Krablite, or baulite, t 

 was then supposed to be a definite silicate, with the atomic ratio 1 : 3 : 24, 

 as determined by the researches of rorclihammer§ and Genth; || but it has 

 since been shown to be a mineral a2:2;res:ate or rock, referred at first to the 



* Bull. Soc. Geol. France, 1S50 (2), vii. 526 ; see also Aim. Mines, 1847 (4), xii. 206, 207 ; 1849, xvi. 

 327, 32S. 



t Jour. Prakt. Cliemie, 1851, lii. 250-258. 



% Laudgrcbe, Miucralogie der Vulcnue, 1870, pp. 60, 227. 



§ Ovcrsigt over det Kgl. dansko Videuskaberues Selskabs Forliandliuger, etc., 1842, pp. 43-55 ; Jour. 

 Prakt. Cliemie, 1843, xxx. 394. 



il Ann. Cliemie PLurm., 1848, kvi. 271. 



