THE NONMETALLIC MINEKALS. 239 



-i. DlASrORE. 



This is ;i hydrous oxide of ahmiiiiuin covrcsponding to the for- 

 mula Al303,H20,=alumina, 85 per cent; water, 15 per cent; hard- 

 ness, 6.5 to 7. It is a whitish, grayish, sometimes l)rownish or yel- 

 lowish mineral, occurring in the form of thin flattened or ai-icular 

 crystals and also foliated, massive and in thin plates or rarely stalactitic. 

 (Specimen No. 53578, U. 8. N. M.) It is transparent to subtranslucent, 

 and sometimes shows violet-blue colors when looked at in one direc- 

 tion, or reddish-blue or asparagus-green in others. Luster, vitreous 

 or pearl3^ 



Occurrence. — The mineral commonly occurs with corundum and 

 emery in dolomite and granular limestone or crystalline schists. In 

 the United States it occurs in large plates in connection with the emery 

 rock at Chester, Massachusetts. 

 Uses. — See under Gibbsite. 



5. Gibbsite; Hydrargillite. 



This is also, like diaspore, a hydrous oxide of aluminum, corre- 

 sponding to the formula AlgO.,, 3H20 = alumina (55.4 per cent, water 

 34.6 percent. The mineral is of a whitish, grayish, or greenish color, 

 sometimes reddish through impurities, and occurs in flattened, hexag- 

 onal crystals, or in stalactitic and mammillary and incrusting surfaces. 

 (Specimen No. 4602, U.S.N.M.). Its occurrence is similar to that of 

 diaspore. 



Uses. — Neither diaspore nor gilibsite have as yet been found in suf- 

 ficient ([uantities to be of economic importance. Should they be so 

 found, their value as a source of alumina is easily apparent. 



6. OCHER. 



The term ocher as commonly used applies to earthy and pulverulent 

 forms of the minerals hematite and limonite, but which are almost 

 invariably more or less impure through the presence of other metallic 

 oxides and argillaceous matter. In nature tht; material rarely occurs 

 in a suitable condition for immediate use, but neecls first to be pre- 

 pared by washing and grinding and perhaps roasting. 



Various varietal names are applied to the ochers, according to their 

 natural colors or sources. The original "Indian red" was a red argil- 

 laceous ocher, with a purplish tinge, found on the island of Ormuz, in 

 the Persian Gulf. A large part of the pigment of this name is now 

 prepared artificially from iron pyrites. Umber is a gray, brown, or 

 reddish variety containing manganese oxides and cW. It derives its 

 . name from Umbria, in Italy, where material of this nature was first 

 utilized. Sienna is a highly argillaceous variety, also from Italy, 

 near Sienna. 



