70 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



LAZURITE. 



Synonym. — Lapis-lazuli. 



Composition. — The ornamental stone known as lapis-lazuli is a 

 mixture of a number of different minerals, but the principal one, 

 yielding the blue color, is lazurite, a sodium aluminum sulpho- 

 ortho-silicate, Na 5 Al 3 S3(Si0 4 )3. 



Crystallization. — Isometric. 



Color. — Deep blue, due to the peculiar sulphur compound present. 



Luster. — Vitreous ; translucent. 



Hardness. — 5; rather too low for the stone to be durable. 



Specific gravity.— 2 A0 ±0.10. 



Optical properties. — Refractive index, 1.49; isotropic. 



The composite nature of the rock can easily be made out by close 

 examination, specks of pyrite in particular being almost always 

 visible. It is decomposed by hydrochloric acid with the evolution 

 of hydrogen sulphide. It occurs in metamorphic rocks. The 

 artificial blue pigment known as ultramarine is essentially identical 

 with lazurite in composition. Blue glass and blue stained chalcedony 

 are often put on the market as imitations. Lapis-lazuli is used in 

 the manufacture of ornaments of various kinds, and, being rather 

 difficult to carve, brings fairly high prices. It does not occur in 

 masses of large size. The only mineral with which it is likely to 

 become confounded is sodalite (see p. 97). 



LIST OF SPECIMENS. 

 CENTRAL ASIA. 



Watch charm carved in imitation of a grasshopper; deep violet-blue; 19.05 

 carats; 40 by 11 by 6 mm No. 356 



CHILE (ANDES MOUNTAINS). 



Slab; oval; deep blue; 33.7 grams (168.6 carats); 65 by 35 by 7 mm No. 355 



PERSIA. 



Cabochon, elliptical girdle; deep blue; 35.57 carats; 30 by 27 by 6 mm. (fig. 



1, pi. 7) No. 354 



Leopardite. — See under Miscellaneous (Porphyry) on page 120. 

 Lintonite. — See under Thomsonite. 



MALACHITE AND AZURITE. 



Composition. — Hydrous copper carbonate, Cu 2 (OH) 2 (C0 3 ). 

 Crystallization. — Monoclinic. 



Color. — Brilliant green, characteristic of many copper compounds. 

 Luster. — Vitreous, or, in fibrous varieties, silky; practically opaque. 

 Hardness. — 3.5; can be used only where not subjected to wear. 

 Specific gravity. — 4 ±0.05. 



Optical properties. — Mean refractive index, 1.8S; double refraction 

 extremely strong, 0.2 ; optically biaxial, negative. 



