72 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 66. 



Stilbite is a common mineral in the veins, being exceeded in 

 amount only by prehnite, apophyllite, laumontite, and datolite. It 

 occurs characteristically as minute colorless transparent crystals and 

 larger groups of parallel crystals or nearly parallel individuals form- 

 ing larger units. These stilbite crystals are associated with all of the 

 other vein minerals and often rest upon prehnite. The smallest of 

 these are all rectangular prisms bounded only by 

 three pinacoids at right angles to each other as shown 

 in figure 24. These become less perfect with increase 

 in size until the larger ones, which are 1 cm. long 

 by 4 mm. wide and 2 mm. thick, show a group struc- 

 ture, the smaller individuals making up the group 

 showing a slight tendency to diverge and form sheaves, 

 although made up of rectangular units. These have 

 a not very pronounced pearly luster on the broader 

 face. One specimen showed minute colorless trans- 

 parent rectangular crystals bristling in all directions 

 from the base to which they are attached and form 

 ing a loose hemisphere. These rest upon a ''moun- 

 tain leather" layer of hornblende and are overlain 

 by laumontite. One of the most perfect of these was 

 measured and gave angles of 90 degrees between the 

 pinacoids, within the limit of error of the measure- 

 ments. These crystals are biaxial, negative, 2V small, 

 r>v weak, |3= 1.498. Lying on the broad face be- 

 tween crossed nicols these show the emergence of the 

 optic normal with a very small inclination, of but a 

 degree or two, of the extinction to the edge, and show 

 a faint suggestion of twinning. When crushed they 

 exhibit two cleavages, one perpendicular to the optic 

 normal or parallel to the plane of the optic axes (010), and a second, 

 nearly as perfect, parallel to the front pinacoid (100) which is per- 

 pendicular to the obtuse bisectrix. This makes the optical orientation, 

 if the crystals are set with their elongation vertical, Y = &, Z = a, 

 Xac=0°-2°. 



Another specimen showing the larger crystals or rather bundles of 

 crystals in parallel position, furnished material for a partial analysis 

 which furnished the results of column 1 of the following table. In 

 column 2 are given the figures of the theoretical composition of stil- 

 bite, taken from Dana, and in column 3 the analysis by Thugutt and 

 Rosicky of epidesmine.-*^ 



Fig. 24.— Stibite of 

 " epidesmine" 

 habit bounded 

 only by three 

 pinacoids. 



2'= Appandix III, Dana Syst. Min., p. 27. 



