2 BULLETIN 415^ U. S, ^EPAETMENT OF AGEICULTUEE. 



COMPOSITION AND PROPERTIES OF ALUNITE. 



Alurdte is a hydrous sulphate of potash and alumina which may 

 be represented by the following formula: 



K2O.3Al2O3.4SO3.6H2O. 



The percentage composition of the pure mineral, according to 

 Dana/ is as follows: 



Per cent. 

 SO3 =38. 60 

 Al203=37. 00 

 KoO =11.40 

 H2O =13.00 



The mineral usually occurs as a massive, fine-grained, pinkish- 

 white rock breaking with a conchoidal fracture. Some of the coarser 

 varieties are distinctly crystalline in character. While theoretically 

 it should contain over 11 per cent of potash, it is seldom found in 

 minable quantities running over 9.5 or 10 per cent in potash. It is 

 frequently contaminated with quartz and the silicates of potash 

 from which it is derived. The mineral is insoluble in water, but 

 soluble in strong sulphuric acid. On heating to a moderate tempera- 

 ture (500° C.) water is driven off and the mineral decomposes into 

 alumina and potassium aluminum sulphate.^ Upon increasing the 

 temperature (to 700° to 750° C.) the latter compound is decomposed, 

 fumes of sulphur dioxide and trioxide are evolved, and a residue 

 remains which consists of alumina and soluble potassium sulphate. 



GEOLOGICAL OCCURRENCE AND ORIGIN. 



Alunite forms seams in trachytic and similar rocks, being produced 

 by the alteration of such rocks by means of sulphurous vapors or 

 sulphate solutions. It is beheved to be closely related in origin to 

 metal veins. 



• Ransome ^ has discussed the various methods by which alunite 

 deposits may be formed, and Butler and Gale * quote extensively 

 from the former in their description of a deposit southwest of Marys- 

 vale, Utah. The latter authors in discussing the Marysvale alunite 

 state that they consider the main vein in this locahty a fissure filling 

 and not a replacement of the wall rock, and cite, as proof of this 

 theory, the fact that the alunite contains but little sihca, while in the 

 wall rock where replacement has taken place quartz phenocrysts 

 remain practically unaltered. They conclude that the mineral vein 

 was introduced in part at least by solutions of deep-seated origin. 



1 System of Mineralogy. 



2 As a matter of fact it is very difficult to drive ofl the water of constitution without also driving ofi some 

 of the oxides of sulphur. 



3 Prof. Paper No. 66, U. S. Geological Survey, pp. 189-195 (1899). 

 < Bull. 511, U. S. Geological Survey (1912). 



