192 DESCRIPTIONS OF MINERALS. 



of muriatic acid depends not only on the amount of oxygen in the ore, 

 but also on the presence or absence of baryta and such other earths as 

 may combine with this acid. The binoxide of manganese, when pure, 

 affords 18 parts by weight of chlorine, to 22 parts of the oxide ; or 23^- 

 cubic inches of gas from 22 grains of the oxide. The best ore should 

 give about three-fourths its weight of chlorine, or about 7,000 cubic 

 inches to the pound avoirdupois. 



Iron ores containing some manganese are used for making spiegeleisen, 

 a hard highly crystallized pig-iron, containing a large amount of car- 

 bon and some manganese. A manganesian iron carbonate or siderite is 

 thus used, and also the franklinite of New Jersey. 



Manganese is also employed to give a violet color to glass. The 

 sulphate and the chloride of manganese are used in calico printing. 

 The sulphate gives a chocolate or bronze color. 



ALUMINUM. 



The aluminum compounds among minerals include only- 

 one oxide — a sesquioxide Al 3 — hydrated oxides, fluorides, 

 and, among ternaries, sulphates, phosphates, and numerous 

 silicates. There are no sulphides or arsenides, and no car- 

 bonate, with a single imperfectly understood exception. 



The silicates are described in the following section. Many 

 aluminum compounds may be distinguished by means of a 

 blowpipe experiment, as explained on page 87. 



Corundum. 



Ehombohedral. R A R or r A r=86° 4'. Cleavage some- 

 times perfect parallel with 0, and sometimes par- 

 allel to the rhombohedral faces. Usual in six- 

 sided prisms, often with uneven surfaces, and 

 sometimes so irregular that the form is scarcely 

 traceable. Occurs also granular. Colors blue, 

 and grayish-blue most common ; also red, yel- 

 low, brown, and nearly black ; often bright. 

 When polished on the surface 0, a star of six 

 rays, corresponding with the six-sided form of 

 the prism, is sometimes seen within the crystal. 

 Transparent to translucent. H. = 9, or next below the 

 diamond. Exceedingly tough when compact. G.=3'9-4'16. 

 Composition. Al 3 = Oxygen 46-8, aluminum 532 = 100; 

 pure alumina. B.B. remains unaltered both alone and with 

 soda. The fine powder moistened with cobalt nitrate and 

 ignited assumes a blue color. 



