DETERMINATION OP MINERALS. 379 



IV. DETERMINATION OP MINERALS. 



Ln" the determination of minerals, no one order in the 

 succession in which characters should be examined answers 

 for all minerals, or even for all of the same section of species. 



A. For species haying a metallic lustre : 



Color will be first noted ; and then streak — that is, the 

 color of the mineral on a surface scratched or abraded by a 

 fine file, or when very finely powdered, and the lustre of the 

 powder or abraded surface, whether metallic like the min- 

 eral or unmetallic. Hardness should be ascertained when 

 obtaining the streak. 



Bloiopipe and chemical characters are of the highest value, 

 giving generally the most certain results. 



Specific gravity is especially distinctive with species hav- 

 ing a metallic lustre, since the differences in density among 

 such species are usually large.* 



Crystalline form and cleavage are of first importance, 

 whenever the specimen allows of their determination. 



B. For species without metallic lustre : 



Streak is sometimes of importance, especially among spe- 

 cies in which it is highly colored. 



Color is generally of little value owing to the variations 

 that frequently come in through impurities. 



Lustre is one of the first characters the eye will observe, 

 but its variation under most species is wide, and often it 

 is of little value. State of aggregation audi fracture for the 

 most part serve to distinguish only varieties. 



Hardness is also often a varying character, the range 

 under some species being from 1 to 6 in the scale of hard- 

 ness ; and still its indications are generally important. 



Crystalline form and cleavage are always important when 

 observable. 



* In using the spiral balance of Jolly (page 65), 1he spiral spring is put at any de- 

 sired height by means of the sliding rod C. The stand B is raised so that the lower 

 San, d, shall be in the water, while the other, c, is above it. The position of the in- 

 ex, or signal, m, is then noted, by sighting across it and observing that the index 

 and the image of it in the mirror are in the same horizontal line ; let s stand for it. 

 Next put the fragment of the mineral in c, and drop the stand B until the lower pan 

 hangs free in the water, and note the position of m, which we may represent by t ; 

 t—s will equal the weight in the air. Now place the fragment in the lower pan and 

 after adjusting again the stand B, the position of m is noted as before ; call it u. 

 Then l—u = loss of weight in water. From these values the specific gravity is at once 

 obtained. 



