198 DESCRIPTIONS OF MINERALS. 
Alums Frequently the sulphuric acid resulting from the oxidation 
of a sulphide, or in some other way, combines also with the iron, 
magnesia or potash or soda of the shale or other rock, as well as the 
alumina, and so makes other kinds of aluminum sulphate. 
Combining thus with potash it produces common alum called Ka/i- 
nite or potash alum, whose formula is K,Al, O,, 8, + 18 aq ; with ain- 
monia, it forms an «ammonia-alum, named ischermigite ; with iron, 
iron alum, called J/alotrichite ; with soda, a soda-alum, Mendozite ; 
with magnesia a magnesia-alum, Pickeringite ; with manganese, a 
manganese-alum, Apjohnite and Bosjemanite. The formulas of these 
alums are alike in atomic proportions, excepting in the amount of 
water, which ¥aries from 18 aq to 24 aq. 
Shale containing alunogen or any of the alums is often called alum 
shale. Such rocks, whether shales or of other kinds, are often quar- 
ried and lixiviated for the alum they contain or will afford. The rock 
is first slowly heated after piling it in heaps, in order to decompose 
the remaining pyrites and transfer the sulphuric acid of any iron sul- 
phate to the alumina and thus produce the largest amount possible of 
aluminum sulphate. It is uext lixiviated in stone cisterns. The lye 
containing this sulphate is afterwards concentrated by evaporation, 
and then the requisite proportion of potassium in the form of the sul- 
phate or chloride is added to the hot solution. On cooling, the alum 
crystallizes out, and is afterwards washed and re-crystallized. The 
mother liquor left after the precipitation is revaporated to obtain the 
remaining alum held in solution. This process is carried on exten- 
sively in Germany, France, at Whitby in Yorkshire, Hurlett and 
Campsie, near Glasgow, in Scotland. Cape Sable in Maryland affords 
large quantities of alum annually. The slates of coal beds are often 
used to advantage in this manufacture, owing to the decomposing 
pyrites present. At Whitby, 130 tons of calcined schist give one ton 
of alum. In France, ammoniacal salts are used instead of potash, 
and an ammonia alum is formed. 
Alum is also manufactured from cryolite (see p. 197), which is ob- 
tained from Greenland. 
Alunite.—Alum Stone. 
Rhombohedral, with perfect basal cleavage. Also mas- 
sive. Color white, grayish, or reddish. Lustre of crystals 
vitreous, or a little pearly on the basal plane. Transparent 
to translucentsi cH. =4.2<G i=2:58—2 7a: 
Composition. K,Al 0. 8;+6 aq=Sulphuric trioxide 3875, 
alumina 3771, potash 11°4, water 13°0=100. B.B. decrepi- 
tates and is infusible ; gives reaction for sulphur. 
Diff. Distinguished by its infusibility, in connection with 
its complete solubility in sulphuric acid without forming a 
jelly. 
Obs. Found in rocks of volcanic origin at Tolfa, near 
Rome ; and also at Beregh and elsewhere in Hungary. 
When it is calcined the sulphates become soluble, and the 
