ART. 18 MINERALS OF ITALIAN MOUNTAIN CROSS AND SHANNON 27 



was to a considerable degree replaced by a fine-grained chalky- 

 appearing white mineral shown by its optical properties to be the 

 adiilaria variety of orthoclase. Epidote, pyrite, titanite, and fine 

 globular grayish chlorite were also deposited in the cavities. (No. 

 84560.) 



Another lot (No. 84562) consists largely of whitish feldspar mixed 

 with more or less pyrite, calcite, epidote, and an undetermined 

 greenish silicate. The massive feldspar has the following optical 

 properties which identify it as anorthite: Biaxial negative ( — ), 

 2V near 90°, dispersion r < v, weak. Kefractive indices a= 1.572, 

 ^=1.580, y=rl.585. It shows a few narrow twin lamellae. Speci- 

 mens Nos. 84560 and 84561 are similar. In a cavity in one specimen 

 of this material are a few simple rhombohedral-appearing crystals 

 up to 5 or 6 mm. in size. These are biaxial negative with 2V large, 

 dispersion r < v, marked ^8= 1.582. These show no twinning and 

 resemble adularia crystals but are shown by their optical properties 

 to also be anorthite. 



ORTHOCLASE 



Small white crystals of rhombohedral appearance and granular 

 masses resting on and apparently replacing anorthite (No. 84560) 

 and associated with epidote, titanite, pyrite — altered to limonite — 

 and a globular gray chlorite are apparently the adularia variety of 

 orthoclase. The adularia is probably older than the epidote and 

 is definitely older than the titanite and chlorite. It is identified by 

 its insolubility in acid and optical properties which are : Biaxial 

 negative ( — ), dispersion r < v strong, fi about 1,520, A=slightly 

 above 1.53. Another specimen of the same number is a mass prin- 

 cipally made up of whitish chalky-looking mineral showing indistinct 

 lusterless crystals of rhombohedral appearance with epidote. The 

 white mineral is biaxial and negative with 2V estimated at 60°. The 

 dispersion, r < v, is perceptible and the indices measured are y8 = 1.531, 

 A= 1,538. This is probably albite. 



Scattered white crystals with the rhombic appearing habit of 

 adularia rest, in one specimen, upon a crust of little green garnets 

 surfacing garnet hornfels. The white crystals, which vary up to 

 3 millimeters in diameter, are optically biaxial negative ( — ) with 

 2V medium, dispersion r < v marked, indices well below 1.55. 

 This mineral is doubtless orthoclase. It is later than the garnet 

 and earlier than the associated pyrite, which is now altered to limon- 

 ite pseudomorphs. 



TITANITE 



Titanite is a common mineral though in small crystals not readily 

 seen without the aid of a lens. It occurs in a variety of situations 

 and varies somewhat in habit. 



