CvVTALOGUE OF GEMS. 489 



rod ill iinother. Luster, vitr(M)us, often weak. Ilaidness. 7.5, .some- 

 what greater than that of quartz; brittle. Specitie gravity. 8.2; cleav- 

 age, prismatic and distinct; often perfect. 



The variety chiastolite, or made, varies in hardness from 5 to 7.5 

 due to the presence of impurities, which, in part, are arranged sym- 

 metrically about the axes of the crystal so as to give a tessellated 

 appearance in cross section. 



Andalusite contains: Silica, 37; ahunina, 63. 



ANHYDRITE. 



VULPINITE. 



Anlu'drite, anhydrous sulphate of lime, has a hardness of 3.5, a 

 specific gravity of 2.0, and a vitreous to pearh* luster. Color, white, 

 grajnsh, bluish, reddish; also brick red and blue. Anh3^drite has been 

 variously denominated muriacite, anhydrite, and tripe stone, according 

 to its structure; the first, when crystallized in l)road lamellae; the 

 second, when granular, and the third, when composed of contorted 

 plates. Vulpinite is a siliceous variety containing 8 per cent of silica 

 and is the kind most used for ornamental purposes. The use of anj- 

 of the varieties is limited. Anhydrite contains 41.2 parts of lime and 

 58.8 parts of sulphuric acid in 1(>(». 



APOPHYLLITE. 



FISH-EYE STONE. 



Apophyllite is occasionally cut for gem purposes. 



The hardness of the mineral is below 5; its specific gravity is 2,33; 

 its color varies from white to gray, occasionally tinged with green, 

 pink, or yellow. It crystallizes in the tetragonal system, usually in 

 octahedrons having their solid angles truncated. The faces thus 

 formed have a decided pearly luster, the others vitreous. Apophyllite 

 was so named in allusion to its tendency to exfoliate under the blow- 

 pipe. Its pearly luster, producing an efiect like that of a fish's e3'e, 

 gave rise to the name "'fish-eye stone," or ichthyophthalmite. 



Apophyllite occurs commoidy as a secondary minei'al in ])asalt and 

 related rocks associated with datolite, pectolite, and the several zeolites. 

 One hundred parts contain: Silica, 52.7; lime, 26; potash. 4.4; water, 

 16.7. There is also a variable proportion of fluorine present which 

 probably bears no relation to the compound. 



AXINITE. 



CrA'stals of axinitc are o<'casioiially cut for ornamental use. Axinite 

 crystallizes in the triclinic system, commonl}^ in broad, acute-edged, 

 wedge-shaped forms, occurring also massive. The hardness of the 

 mineral is about 7, or near that of (piartz, Init it is so extremely 

 brittle that it is fashioned only with difficulty. Luster, vitreous. 



