50 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



FELDSPAR. 



Varieties. — This name includes several varieties, which are dis- 

 tinguishable by their crystallization and chemical composition, 

 comprising albite, containing sodium (Na); labradorite, sodium 

 and calcium (Na+Ca); microcline, potassium (K); oligoclase, so- 

 dium and calcium (Na-f-Ca); and orthoclase, potassium (K); and in 

 addition several varieties based on peculiarities of structure or color, 

 as amazonstone, adularia, moonstone, perthite, and sunstone. 



Composition. — Silicates of potassium or sodium; in part, combina- 

 tions of silicates of sodium with ortho-silicates of calcium, the element 

 aluminum being present throughout. KAlSi 3 8 , NaAlSi 3 8 , and 

 mNaAlSi 3 8 + nCaAl 2 (Si0 4 ) 2 . 



Crystallization. — Monoelinic or triclinic. 



Color. — Colorless when pure; sometimes colored pink or green by 

 impurities of unknown nature; in addition, may show internal 

 reflection colors of two types, the one due to the presence of innumer- 

 able minute laminae, the other due to inclusions of hematite. 



Lust°r. — Vitreous; transparent to translucent. 



Hardness. — 6; not very durable. 



Specific gravity. — 2.5 to 2.7. 



Optical properties. — Mean refractive index, 1.53, varying from one 

 kind of feldspar to another; optically biaxial. 



The feldspars are distinguished by their optical properties, moder- 

 ate hardness, pronounced cleavage, and peculiar color phenomena. 



They are cut for the most part cabochon, to bring out color effects, 

 and are classed as semiprecious stones. Only exceptionally fine 

 stones are worth more than the cost of cutting. They are shown to 

 best advantage in the varieties moonstone, orthoclase, and amazon- 

 stone. 



LIST OF SPECIMENS. 



FELDSPAR, variety ADULARIA. 



SWITZERLAND (ST. GOTTHARD). 



Three cabochons, two elliptical, one rectangular girdle; colorless; 2.175, 2.09, 

 and 0.G85 carats; two 10 by 9 by 4 mm. ; one 7 by 5 by 2 mm No. 405 



FELDSPAR, variety AMAZONSTONE. 



SIBEKIA. 



Rectangular slab; green; 110.45 carats; 72 by 40 by 4 mm. Gift of Clarence 

 S. Bement - No. 387 



Three gems, two cabochon, one double cabochon; elliptical girdle; green 

 and blue-green; 9.13, 6.77, and 5.03 carats; 22 by 17 by 4 mm., 17 by 13 by 

 4 mm., 15 by 11 by 5 mm No. 389 



Cabochon, elliptical girdle; green; 5.79 carats; 18 by 13 by 3 mm. Isaac Lea 



collection No - 388 



