530 AUSTIN F. ROGERS 



The crystalline mineral corresponding to cliachite is hydrargil- 

 lite or gibbsite. This occurs to a minor extent in pisolitic bauxite 

 and also in some clays, but in the "granitic type" of bauxite from 

 central Arkansas 1 hydrargillite is the principal mineral. 



The composition of bauxite is usually given as A1 2 3 *2H 2 0, 

 but, as in the other amorphous minerals, the water content 

 of cliachite is variable. In the purest of the Georgia bauxites 

 the ratio of A1 2 3 to H 2 is very close to i to 3, according to 

 Watson. 2 



An amorphous mineral from India described by Warth 3 under 

 the name gibbsite has the composition A1 2 3 *3H 2 and is probably 

 cliachite. 



Shanyavskite, an amorphous mineral with the composition 

 A1 2 3 '4H 2 0, is probably essentially the same as cliachite. 



I have examined several specimens of cliachite and have found 

 the index of refraction to be 1.570=1=0.005. 



Pyrolusite. Mn0 2 (H 2 0) s . — Pyrolusite is probably an amor- 

 phous manganese dioxid corresponding to crystalline polianite. 

 The fibrous structure is due to the fact that it is pseudomorphous 

 after manganite. It always contains a small amount of water. 



Psilomelane. Formula doubtful (wad, lithiophorite, asbolane, 

 lampadite). — Lacroix says that psilomelane is crystalline. There 

 may be crystalline equivalents of psilomelane, such as hollandite 

 and romanechite, but most specimens of psilomelane show no indi- 

 cation of crystalline structure and are doubtless amorphous. 

 Psilomelane is probably a salt of some manganese acid and not 

 simply an oxid of manganese. Along with water it may contain 

 BaO, CaO, MgO, Fe 2 3 , A1 2 3 , K 2 0, Na 2 0, Li 2 0, CoO, and CuO. 

 A specimen from near Sodaville, Nevada, presented to me by 

 Mr. L. B. Spencer, is said to contain Sn0 2 and W0 3 . 



Collophane. 3Ca 3 (P0 4 ) 2 - Ca(C0 3 ,F 2 ) (H 2 0)^ (apatite in part, col- 

 lophanite, fluocollophanite, quercyite, nauruite). — The principal 

 constituent of phosphorite or so-called phosphate rock is not mas- 

 sive apatite, but an amorphous substance which is identical with 



1 Mead, Econ. Geol., X (1915), 41. 

 'Am. Geol., XXVIII (1901), 25. 

 3 Mineral. Mag., XIII (1902), 172. 



