80 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



PREHNITE. 



Variety. — Chlorastrolite. 



Composition. — Hydrogen calcium aluminum ortho-silicate, 

 H 2 Ca,Al,(Si0 4 ) 3 . 



Crystallization. — Orthorhombic. 



Color. — White when pure, but usually pale green, probably owing 

 to the presence of iron. The variety chlorastrolite is mottled green 

 and white. 



Luster. — Vitreous; in chlorastrolite somewhat silky; translucent. 



Hardness. — 6.5; too hard to be scratched readily by a knife; fairly 

 durable. 



Sp< cijic gravity. — 2.90 ± 0.10. 



Optical properties. — Mean refractive index, 1.63; double refrac- 

 tion strong, 0.03 ; optically biaxial, positive. 



The mineral is distinguished by its hardness and optical proper- 

 ties from other minerals that resemble it. It is found in veins and 

 cavities in basic igneous rocks, especially basalt and diabase. 



Ordinary prehnite is rarely cut as a precious stone, as its color is 

 too pale to be attractive. The variety chlorastrolite, which is mostly 

 obtained from the beaches of Lake Superior, is cut cabochon to bring 

 out the fibrous structure, and is used in scarf pins, etc. It brings but 

 a moderate price, only enough to pay for handling. 



LIST OF SPECIMENS. 



UNITED STATES. 



New Jersey. 



Hoxie's Quarry, Paterson, Passaic County: 



Cabochon, rectangular girdle; light green; 49.8 carats; 54 by 14 by 8 



mm No. 361 



Two gems, cabochon, circular girdle; light green; 11.98 and 11 carats; 



17 by 6 and 17 by 5 mm No. 362 



PREHNITE, variety CHLORASTROLITE. 



Michigan. 

 Isle Royal, Lake Superior: 



Cabochon, elliptical girdle; dark and light green , mottled; 12 carats; 20 



by 17 by 4 mm. (fig. 2, pi. 7) No. 364 



Cabochon, elliptical girdle; dark and light green, mottled; 5.43 carats; 



12 by 8.5 by 6 mm No. 363 



Four polished stones, irregular; dark and light green, mottled; largest, 

 7.7 carats; total weight of three small stones, 3.29 carats; largest 16 by 

 11 by 7 mm No. 366 



Three polished stones, two cabochon, one double cabochon; gray-green 

 with dark green spots; 6.035, 4.87, and 3.67 carats; 17.5 by 12 by 3, 14 

 by 11 by 4, and 13 by 8 by 4.5 mm. Isaac Lea collection No. 365 



