THE NUNMETALLIC MINERALS. 417 



It is obtained jirtiticiully from the waste of jifas works. Mendozite is a 

 soda alum of the composition Na2S(\.Al.,(SOj3+24:H.,(), = sodium sul- 

 phate, 15.5 per cent; aluminum sulphate, 87.3 per cent; water, 47.2. 

 The mineral closely resembles ordinary alum, and has been reported 

 from Mendoza, in the Argentine Re})u)>lic, hence the name. Picker- 

 ingite is a magnesium alum of the composition MgSO^.Al.^(SOj3+22 

 HgO, = aluminum sulphate, 30.9 per cent; magnesium sulphate, 14 per 

 cent; water, 40.1 per cent. The mineral is of a white, yellowish, or 

 sometimes faintly reddish color, of a bitter, astringent taste, and 

 occurs in acicular crystals or fibrous masses. (Specimen No. 63043, 

 U. S. N. M., from Tarapaca, Chile.) Halotrichite has the composition 

 FeSO^. AI.3 (SO.,)3+24H20, =ahuninum sulphate, 36.9 per cent; ferro.us 

 sulphate, 16.4 per cent; water, 46.7 per cent. The mineral is of a 

 white or yellowish color, and of a silky. til)rous structure, hence the 

 name from the Greek word aXg, salt, and OpiS, a hair. Apjohnite has 

 the formula MnSO^.Alo(SO^).j + 24H20, — manganese sulphate, 16.3 per 

 cent; aluminum sulphate, 37 per cent: water, 46.7 per cent. It occurs 

 in silky or asbestiform masses of a white or yellowish color, and tastes 

 like ordinary alum. It has been found in considerable quantities in 

 the so-called "alum cave" of Sevier County, Tennessee. According 

 to Safi'ord:^ 



This is an open place under a shelving rock. * * * The slates around and 

 above this contain much pyrites, in fine particles and even in rough layers. * * * 

 The salts are formed above and are brought down by trickling streams of water. 

 * * * Fine cabinet specimens could be obtained, white and pure, a cubic foot in 

 volume. 



Dana states that the cave is situated at the headwaters of the Little 

 Pigeon, a tributary of the Tennessee River, and that it is properly an 

 overhanging cliff 80 or 100 feet high and 300 feet long, under which 

 the alum has collected. It occurs, according to this authority, in 

 masses, showing in the cavities fine transparent needles with a silky 

 luster, of a white or faint rose tinge, pale green or yellow. Epsomite 

 and melanterite occur with it. Alunogen has the composition AI2 

 (SOjj+lSHgO, = sulphur trioxide, 36 per cent; alumina, 15.3 per 

 cent; water, 48.7 per cent; hardness, 1.5 to 3; specific gravity, 1.6 

 to 1.8. This is a soft white mineral of a vitreous or silky luster, 

 soluble in water, and with a taste like that of the common alum of the 

 drug stores. It occurs in nature both as a product of sublimation in 

 volcanic regions, and as a decomposition product from iron pyrites 

 (iron disulphide) in the presence of aluminous shales. So far as the 

 present writer is aware, the native product has no commercial value, 

 })eing found (on account of its readv solubility) in too sparing quanti- 

 ties in the humid East, while the known deposits in the arid regions 

 are remote and practically inaccessible. A white fibrous variety is 



' Geology of Tennessee, 1869, p. 197. 

 NAT MUS 99 27 - 



