MINERALS FROM LYON MOUNTAIN, CLINTON COUNTY *J\ 



Stilbite 



Stilbite occurs abundantly in drusy crusts and aggregates of 

 colorless transparent crystals and in yellow to brown sheafs, all of 

 which display the characteristic pearly luster on the clinopinacoid. 

 The planes of m (no), usual in the twinned parallel grouping of 

 stilbite, are here replaced by those of. the hemiorthodome / (101) 

 which with the pinacoidal planes c (ooi) and b (oio) produce a 

 combination varying but slightly in shape from a right parallel- 

 opiped, and which give to the parallel groupings a flat rather 

 than a serrated aspect. This parallel grouping is shown in figure 

 21. The usual penetration twins with the twinning plane parallel 

 to c are apparent in sections parallel to b in polarized light. 



Biotite 



Biotite in distinct crystals occurs imbedded in calcite of type I 

 from a vug opening into the 600-foot level. The crystals average 

 20 mm in diameter and show fair development of the planes of /x 

 (in) and b (010). A marked twinning habit with c for the com- 

 position face was noted in a number of instances, the arrangement 

 of the crystals being that shown in figure 22. No accurate readings 

 could be obtained in the reflection goniometer owing to the dull 

 and irregular character of the faces, but sufficiently close measure- 

 ments were reached with the contact goniometer to identify the 

 forms b (010) and /* (in). A decided tapering toward the vertical 

 termination due to "stepped" crystals in parallel position is charac- 

 teristic of the occurrence. The crystals are black in color and only 

 transparent in very thin plates. The interference figure in con- 

 vergent light shows a small axial angle. 



Titanite 



Titanite of the variety lederite was obtained from the walls of the 

 largest "vug" opening into the 600-foot level. The crystals which 

 measured 3 to 15 mm on the b axis occur associated with ortho- 

 clase, magnetite and quartz of the first generation. They are dark 

 brown to black in color and show brownish red in thin sections by 

 transmitted light. A distinct parting parallel to 77 (221) gave a 

 measured angle of 125 35' corresponding to a calculated value 

 rjAr)' 1 2 5 42'. The prevailing crystal habit is shown in figures 

 2 3a-2 3b, although in one instance a considerable development of 

 the planes of n (in) produced an elongation parallel to these planes 

 which simulated a prismatic habit. The observed planes lie mostly 



