AKT. 18 MINERALS OF ITALIAN MOUNTAIN CROSS AND SHANNON 35 



scolecite (No. 84580). The latter mineral forms radiating hemi- 

 spheres, up to 1 cm. in diameter, of fine fibers of snow-white color. 

 Under the microscope this fine fibrous material is somewhat opaque 

 from included air but has negative elongation and apparently small 

 inclined extinction with an index of refraction approximating 1.510. 

 The age relation of scolecite and stilbite seems reversible, some- 

 times one and sometimes the otlier being oldest. 



A sample of scolecite was analyzed by Eakins, but which speci- 

 men furnished the material is not known. None of the specimens 

 was originally catalogued as scolecite. The anal3'sis gave the fol- 

 lowing results : 



Analysis of scolecite 



SiOi. 45.90 



AI2O3 - 26. 51 



FCiOa 



CaO 14. 17 



MgO Trace 



Na^O Trace 



H2O plus 13. 79 



Total 100. 37 



The specific gravity of the analyzed material is given as 2.247 

 at 17.2° C. The results of the analysis are almost in exact agree- 

 ment with the theoretical composition of scolecite as given by 

 Dana. 



THOMSONITE 



One specimen of this mineral forms a vein in altered diorite some 

 3 cm. in thickness. This is bordered by a 2 to 3 mm. crust of black- 

 ish green sahlite and is filled with broad plates of glistening specu- 

 larite and a white zeolite in radiating pearly blades. Under the 

 microscope the latter is colorless and free from alteration. It has 

 a perfect cleavage upon which the plates lie and the acute bisectrix 

 is perpendicular to this cleavage. The mineral is biaxial and 

 positive (-f ) with 2V=50 to 60°, dispersion r < v, weak. The 

 refractive indices are approximately, a=1.515, jS=1.520, y=1.538, 

 Birefringence =0,023. These properties indicate that the mineral is 

 a variety of thomsonite. 



STILBITE 



Although less abundant that scolecite and chabazite, stilbite is 

 common in the materials, usuaUy as comparatively inconspicuous 

 crystals, drusy crusts, or rosettes. In one or two specimens it is 

 present as sheaves of such size and quality as to make fair stilbite 

 specimens. 



