50 Washington — Study of the Glaucophane Schists. 



Si0 2 47-84 



A1 2 3 16-88 



Fe 2 3 .. 499 



FeO ._. 5-56 



MgO 7-89 



CaO 11*15 



Na 2 3-20 



K 2 0-46 



H 2 105°+ 1-81 



H 2 105°- 0-17 



PA---- 0-14 



MnO 0-56 



100-65 



Garnet-glaucophane Schist. Pine Canyon, Mount Diablo. 

 Melville anal. Bull. Geol. Soc. Amer., vol ii, p. 413, 1891. 



zoisite-glaucophane schists. One of the last was analyzed, and 

 is described by Becker as follows. 



"It is a greenish-gray, schistose rock, consisting chiefly of 

 glaucophane and zoisite. The latter occurs in imperfect pris- 

 matic crystals. The zoisite is distinctly dichroic and has a 

 faint olive-green tint when the prisms are parallel to the prin- 

 cipal section of the nicols. Sections perpendicular to the main 

 axis are square, often with one truncated corner. The extinc- 

 tions are normal and the colors of interference are gray to 

 yellow in the prisms, but more vivid in granular aggregates. 

 The angle of the optical axes appears to be large. Glauco- 

 phane, in needles and long, imperfect, nearly parallel prisms, 

 gives the rock its cleavage. Quartz, albite, muscovite and 

 titanite are present." 



The occurrence of a purely zoisite-glaucophane schist, as 

 described by Becker, is decidedly unusual, and the determina- 

 tion of all the supposed zoisite as such would seem to be some- 

 what problematical. It is true that some of the mineral was 

 separated and analyzed,* the results corresponding fairly well 

 with the determination as zoisite, but being not inconsistent 

 with an epidote low in iron. The color and pleochroism are 

 also unusual for zoisite. It would seem probable that both 

 minerals exist in the rock, the granular form being probably 

 especially referable to epidote. 



Melville's analysis (I) shows a composition closely resembling 

 the preceding one, and like other glaucophane schist analyses, 

 with high CaO. It is of interest to compare with it an analysis 

 of a diabase (Becker's pseudodiabase) from Mt. St. Helena, also 

 by Melville. It will be seen that the two are almost identical. 



* Becker, op. cit., p. 79. 



