2 BULLETIN 415, U. S. DEPARTMENT OF AGRICULTURE. 
COMPOSITION AND PROPERTIES OF ALUNITE. 
Alunite is a hydrous sulphate of potash and alumina which may 
be represented! by the following formula : 
K 2 0.3A1 2 3 .4S0 3 .6H 2 0. 
The percentage composition of the pure mineral, according to 
Dana, 1 is as follows: 
Per cent. 
S0 8 
=38. 60 
ALA 
=37. 00 
K ? 
=11. 40 
H 2 
=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 aoid. On heating to a moderate tempera- 
ture (500° C.) water is driven off and the mineral decomposes into 
alumina and potassium aluminum sulphate. 2 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 believed to be closely related in origin to 
metal veins. 
Ransome 3 has discussed the various methods by which alunite 
deposits may be formed, and Butler and Gale 4 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 locality 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 silica, 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 off the water of constitution without also driving off some 
of the oxides of sulphur. 
a Prof. Paper No. 66, U. S. Geological Survey, pp. 189-195 (1899). 
4 Bull. 511, U. S. Geological Survey (1912). 
