520 MINERALOGY 



General description. Crystals are thin plates or tabular; also 

 in foliated and micaceous aggregates. Chemically some arsenic may 

 replace the phosphoric acid. Zeunerite, Cu(U0 2 )2(AsO 4 ) 2 . 8 H 2 O, 

 is the arsenic mineral isomorphous with torbernite and very simi- 

 lar to it, except in color, which is lemon or sulphur yellow. 



Autunite, Ca(UO 2 )2(PO 4 ) 2 . 8 H 2 O, is a member of the same 

 group, but orthorhombic and lemon or sulphur yellow in color. 



All three minerals are secondary oxidation products associated 

 with uranium deposits of Joachimsthal, Bohemia. Torbernite is 

 associated in small amounts with the carnotite at Richardson, 

 Utah. 



NITRATES 



SODA NITER 



Soda Niter. Chili Saltpeter, NaNO 3 ; Nitrate of soda ; Na 2 

 = 36.5, N 2 O 6 = 63.5 ; Hexagonal ; Type, Dihexagonal Alternat- 

 ing; c = .8276; 0001 A 1011 = 43 42'; r* r' = 73 30'; Cleavage, 

 rhombohedral perfect ; Fracture, conchoidal ; Brittle; H. = 1.5-2; 

 G. = 2.24-2.29 ; Color, white, gray, red, brown, or yellow ; Trans- 

 parent; o) = 1.587; = 1.336; o> - e = .251; Optically (-). 



B.B. Deflagrates on coal, colors the flame intensely yellow 

 (Na). Has a cooling taste, easily soluble in water and yields reac- 

 tions for nitrogen, page 590. 



General description. Crystals are rare, usually in beds, crusts, 

 or granular. It is isomorphous with calcite, though differing 

 from it entirely chemically. All nitrates are very soluble in water, 

 and their occurrence in nature is therefore restricted to arid regions 

 or to caves where little water has access. Nitrates are formed in 

 the soils by the oxidation of organic matter through the action of 

 certain bacteria. Nitrates are carried in the ground water and 

 serve as a supply for growing vegetation. Nitrogen is one of the 

 most expensive as well as the most important plant foods, and for 

 this reason the nitrate deposits of Chili are of enormous commercial 

 importance, as they are the only extensive deposits of nitrogen 

 salts in the world. The origin of these beds has as yet not been 

 satisfactorily explained. They may have been deposited by 

 evaporating solutions, by volcanic action, or by decaying organic 

 materials. They extend over an area of many square miles in 



