AKT. 2. 



PETROLOGY AT GOOSE CEEEK SHANNON. 



43 



Forms and angles of albite from miarolitic cavity. 



Quality de- 

 scription. 



Poor 



Excellent 



Fair 



Good 



Medium _ 



Measured. 



61 21 



120 26 



82 20 



150 38 



90 09 



90 09 



27 12 



90 00 



!135 12 ! 34 10 



Calculated. 



60 30 



119 52 



81 51 



149 44 



135 21 



90 00 



90 00 



27 01 



90 00 



34 11 



No reflection. Identified b3^zonal position. 



The crystals tend to aggregate in parallel position. They are 

 twinned on the albite law. On many of them the 

 twinning is not conspicuous under a lens as the lam- 

 inae in reverse position are very thin, but a few are 

 composed of two individuals, each of them complete, 

 united in alternate positions on the (010) face. 



When these crystals are crushed and examined 

 under the microscope they are found to be clear and 

 transparent with a mean refractive index of 1.530 

 to 1.531, indicating a nearly pure albite. 



Diopside, which is absent from many of the cav- 

 ities, abuts against some as though by accident of 

 position. In a few, usually in altered albitic peg- 

 matite, it forms v»"ell developed transparent bottle 

 green crystals which are evidently deposits in the 

 cavity rather than diopsides of the surrounding rock 

 which have chanced to abut against the cavity. 

 Many of the diopsides have a terminal outgrowth of 

 silky fibers of hornblende in parallel position or as fig.3.-albite-show 

 loosely attached tufts and the cavity containing the 

 largest and best diopside crystals had its central por- 

 tion completely filled with a matted aggregate of 

 the silky hornblende. A crystal from this cavity, 3 mm. b}'- 7 mm. 

 in size, was measured, yielding the following measurements: 



Forms and angles of diopside crystal from miarolitic cavity. 



ING COMMON HABIT 

 OF ALBITE CRYSTALS 

 OCCUKRING IN MIA 

 ROLITIC CAVITIES. 



