196 DESCRIPTIONS OF MINERALS. 



colors from red to brown and black : especially at Frank- 

 lin. Xewton and Sparta, in the former, and in Warwick, 

 Amity and Edenville, in the latter. The crystals are octa- 

 hedrons, and often grouped or disseminated singly in gran- 

 ular limestone. One crystal, found at Amity by Dr. Heron, 

 weighs 49 pounds. The limestone quarries of Bolton, Box- 

 borough, Chelmsford and Littleton, Mass., afford a few 

 crystals. 



Crystals of spinel are occasionally soft, having under- 

 gone a change of composition approaching steatite in all 

 characters except form. They are true pseudomorpJis. They 

 are met with in Sussex and Orange counties. Other spinel 

 pseudomorphs consist of hydrotalcite (see preceding page). 



Uses. The line colored spinels are much used as gems. 

 The red is the common ruby of jewelry, the oriental rubies 

 being sapphire. 



Gahnite is a spinel in which zinc takes the place of part or all of the 

 magnesium; when all, it is called Aiitomolitc. Color dark green or green- 

 ish black. H. =7*5-8. G. — 4-4-G. When fused with sufficient soda, 

 B.B. on coal a white coat of zinc oxide is deposited, which is yellow 

 when hot. B.B. infusible. At Franklin, X. J., and at the Canton 

 mine in Georgia. Occurs in granite at Haddam with beryl, chryso- 

 beryl. garnet, etc. In Sweden, near Fahlun. in talcosc slate. 



Dysluite. A variety of gahnite containing oxide of manganese. 

 Color yellowish or grayish-brown. H. =7 - 5-8. G. =4oo. Composi- 

 tion, Alumina 30*5, zinc oxide K3 - 8, iron sesquioxide -il'9, manganese 

 protoxide TO, silica 3, water 4. From Sterling, X. J., with frank- 

 linite and troostite. 



Kreittonitc is a zinc-iron gahnite. 



Hercinite is a spinel affording on analysis alumina and iron protoxide, 

 with only 2 9 per cent, of magnesia. 



Chrysoberyl. 



Trimetric. 7a/=129 c 38'. Also in compound crystals, 

 as in fig. 2. Crystals sometimes thick ; often tabular. 



Color bright green, from a light shade to emerald-green ; 

 rarely raspberry or columbine-red by transmitted light. 

 Streak un colored. Lustre vitreous. Transparent to trans- 

 lucent. H.=:8-5. G. =3-5-3-8. 



Composition. BeAl 0, = Alumina 80*2, glucinal9'8=: 100. 

 A little iron is sometimes present. B.B. infusible and un- 

 altered. 



Alexandrite is an emerald-green variety from the Urals, 

 colored by chrome, bearing the same relation to ordinary 

 chrysoberyl as emerald to beryl. Fig. 7 is of this variety. 



