USEFUL J/AT.1/.N 219 



Tin- .iliiiniiium minerals may be divided into five di-hn.-t 

 classes sollows: (1) Those used for the extraction of tin- metal; 

 <( irundum, cryolite, bauxite and gibbsite. (2) Those used din < tly 

 as gems; ruby, sapphire and turquoise. (3) Those used as an 

 abrasive on account of their superior hardness; corundum, emery 

 and diaspore. (4) Those known directly as alums or used in the 

 manufacture of alums; alunogen and alunite, together with many 

 nat ural alums. (5) The anhydrous and hydrous silicates bearing 

 aluminum. 



Aluminum occurs as an essential constituent of all important 

 rocks, save the limestone, marbles, dolomites and saml-inm ~. 

 and even here it is a common impurity. It is present in all the 

 micas and feldspars that have wide industrial application. It is 

 a necessary constituent of all clays. 



Origin of the Ores. Corundum and its varieties occur both as 

 primary and secondary minerals. According to F. W. Clarke 

 the most important experiments from a geological standpoint 

 upon the genesis of corundum have been conducted by Moro/e- 

 wicz. Morozewicz worked indefatigably upon large artificial 

 magmas, using the furnace of a glass factory in the preparation of 

 his melts; and he found that whenever the alumina in comparison 

 with the other bases exceeded a certain ratio, the excess upon 

 cooling crystallized out completely as corundum, as spinel, as 

 sillimanite, or as iolite, dependent upon percentages. Previous 

 to the appearance of Morozewicz's memoir, corundum was not 

 supposed to be a pyrogenic mineral but a product of contact 

 matamorphism, especially in limestones. (See Fig. 116.) 



According to J. H. Pratt corundum occurs in North Carolina 

 and Georgia as a pyrogenic mineral from the fractional crystal- 

 lization of a peridotite magma, rich in the ferromagnesian mineral 

 olivine. Corundum has been observed as a primary mineral in 

 both pegmatites and syenites, but these occurrences are rare. 



Corundum, of secondary origin, is a product of contact 

 metamorphism and is generally associated with shales and lime- 

 stones. Emery is perhaps best regarded as a variety of corundum. 

 It consists of corundum admixed with hematite, magnetite, and 

 sometimes spinel. 



The hydrous oxides, diaspore and gibbsite are often formed by 

 the hydration of corundum. Bauxite, according to C. W. Hayes 

 belongs to the hot spring deposits. Percolating meteoric water- 

 have converted pyrite, the sulphide of iron, into melanterite. 



