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 Kali- 

 niie or potash alum, whose formula is K 2 A1 3 24 S 4 + 18 aq ; with am- 

 monia, it forms an ammonia-alum, named 'ischermigite ; with iron, 

 iron- alum, called Halotrichite ; with soda, a soda-alum, Mendozite ; 

 with magnesia a magnesia-alum, Pickeringite ; with manganese, a 

 manganese-alum, Apjohnile and Bosjemanite. The formulas of these 

 alums are alike in atomic proportions, excepting in the amount of 

 water, which varies from IS 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 next 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, witli 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 translucent. H. =4. " G. = 2 , 5S-2 , T5. 



Composition. K 2 Al 22 S 4 + 6 aq= Sulphuric trioxide 38*5, 

 alumina 37*1, potash 11 % 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 



