264 MINERALOGY 



Where only an approximate determination is all that is required, 

 as in the identification of minerals, the fragment is placed on the 

 liquid, and the liquid is diluted until suspension is reached, then 

 small fragments of minerals, the specific gravity of which are known, 

 are used as tests. Thus if calcite sinks and quartz floats, the spe- 

 cific gravity of the mineral to be determined must lie between 2.65 

 and 2.72 or not far from 2.70. 



A list of convenient test minerals is as follows : 



Heulandite 2.20 Calcite 2.72 



Analcite 2.26 Prehnite 2.87 



Gypsum ....... 2.32 Datolite 2.95 



Leucite 2.47 Tremolite 3.00 



Orthoclase 2.56 Actinolite 3.10 



Quartz 2.65 Fluorite 3.18 



When accuracy is required the Westphal balance, Fig. 376, is 

 used. This consists of a bob n which just balances the arm H 

 in air. The arm is divided into a decimal scale. Three sizes of 

 weights are furnished with the instrument, A = units, B = .1 

 and C = .01, when hung on the hookas A 2 , and corresponding tenths 

 of these values when on the arm. 



In the determination, the bob is immersed in the liquid and the 

 balance arm weighted to balance, when the specific gravity is read 

 directly from the weights and the arm. 



The separation of minerals with heavy solutions. The different 

 mineral components of a rock such as granite may be separated by 

 the use of heavy liquids. The specimen is ground only sufficiently 

 fine to insure each particle as consisting of one mineral species 

 only ; the fineness necessary is determined by an examination of 

 the powder with a microscope. The sample is then charged in 

 chamber A of the separatory apparatus, Fig. 377, and Thoulet's 

 solution, density 3.00, poured in, stoppered and shaken; all acces- 

 ^ory minerals, as zircon, apatite, magnetite, amphibole, and tour- 

 maline, will settle in the chamber A, while the others will float on the 

 surface of the liquid. The heavy minerals may, after settling, be 

 drawn into B by means of the cock C, and finally after shaking a 

 second time, out at the bottom by turning the cock C'. The solu- 

 tion in chamber A is now diluted until calcite just sinks, when it is 

 again shaken, allowed to settle, and all minerals above 2.72, as the 

 micas, are drawn off. The solution in A is again diluted until 

 quartz just sinks, which is separated from the orthoclase. 



