334 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



irregular grains from 1 to 2 mm. in diameter which make up approxi- 

 mately half of the rock, the other major constituent being crystalline 

 calcite. The chondrodite shows no definite crystal faces when the 

 calcite is removed by solution in acid. In the hand specimen it appears 

 as grains of the usual resin-brown color. Under the microscope in 

 powder the chondrodite is clear, transparent, and devoid of cleavage; 

 is yellow-brown in color and distinctly pleochroic X = rich golden 

 brown. Y = pale brownish yellow. Z = verv pale brownish yellow: 

 absorption. X>Y>Z. It is biaxial positive ( + ) with 2V medium 

 large, dispersion, r>v, distinct. Refractive indices a= 1.631,. 

 13=1.040. 7 = 1.657, all ±0.005, birefringence. 7-0. = . 026. Asso- 

 ciated with the chondrodite are a deep bottle green isotropic spinel 

 with a refractive index of 1.735 ± .002. and less of a colorless micaceous 

 mineral which is nearly or quite uniaxial and positive with a refractive 

 index of approximately - 1.598. This is doubtless clinoehlore. 



ILVAITE (417) 



LIEVRITE, YENITE 



Hydrous lime-iron silicate, H 2 O.CaO. Orthorhombic. 



*4FeO.Fe : 3 .4SiO : . 



The rare calcium-iron silicate, ilvaite, has been found in the South 

 Mountain mining district in Owyhee County in crystals which, in 

 size and perfection, are equal to the best known from any locality 

 in the world. 60 



OWYHEE COUNTY 



The ilvaite occurs in the ores of the Golconda and perhaps other 

 mines in typical eontaet-metamorphic deposits. A large area of 

 tine grained granite or granodiorite here adjoins an area of highly 

 metamorphosed sedimentary rocks, largely schists, quartzites, and 

 marble. In a broad band of marble near the granite there is a series 

 of masses of eontaet-metamorphic minerals including hedenbergite, 

 garnet, ami ilvaite. carrying in some places pyrrhotite. chalcopyrite, 

 sphalerite, and galena. The minerals are aggregated in the irregular 

 manner of eontaet-metamorphic deposits, the ilvaite being in bunches 

 scattered through the masses. The specimens of ilvaite consist of im- 

 perfectly crystallized black masses associated with granular reddish- 

 brown garnet : of unterminated crystals having good prismatic planes, 

 embedded in quart/.: of large crystals embedded in calcite patches in 

 a tine-grained hedenbergite rock: and of large and tine terminated 

 prismatic crystals embedded in soft limonitic gossau (pi. S, upper*. 



The unterminated crystals embedded in quartz are commonly 

 prismatic and are deformed by mutual interference but some of 



rl V. Shannon. Amor. Journ. Sci., vol. 4". p. 118, 191$. 



