jt; W. Mallet on a Zeolitic Mineral. 



179 



Art, XIV. — On a Zeolitic mineral {allied to Stilbite) from the Isle 

 of Skye, Scotland; by J. W. Mallet, Ph.D. 



The specimen to which the following description refers has 

 been in my possession for several years, and has attached to it a 

 label bearing the name " Hypostilbite," but analysis shows it to 

 be a mineral quite distinct from Beudant's hypostilbite of the 

 Faroe Islands, and differing also from both stilbite proper and 

 epistilbite. 



It occurs as a mass of minute crystals, resembling white loaf 

 sugar, breaking easily, and crushing under the fingers into a 

 coarsish crystalline powder. The separate grains viewed under 

 the microscope appear as single prismatic crystals or little groups 

 of three or four, nearly transparent, colorless, and with a pearly 

 lustre, especially on two opposite faces, — closely resembling stil- 

 bite in fact in general appearance. 



The crystalline form could not be satisfactorily made out, but 

 seemed to be monoclinic. Hardness a little greater than that of 

 calcite. Specific gravity —2 252. 



Strong muriatic acid poured over the pulverized mineral at 

 night had the next morning formed a distinct jelly. 



On analysis the following results were obtained. 



Atoms. 



Silica, 53-95 1-191— 8- 



Alumina, 20*13 -892—1- 



Lime, 12*86 459—1*17 



Magnesia, trace 



Potash (with a little soda), *87 



Water, 1242 1*380—3-52 



100;23 



Neglecting the small quantity of alkali, these numbers lead 

 us nearly to the formula, 



2(CaO, SiOs)+2(Al a 09, 2Si0 3 )+7HO, 

 which differs completely from that of stilbite, CaO, SiCh +AI2 

 Oa, 3SiOs + 6HO, or that of epistilbite, CaO, SiOs -f AI2O3, 

 3Si03+5HO. The percentage of water is also far too small for 

 hypostilbite. 



The mineral appears to be a distinct one, and does not seem 

 to have resulted from the gradual decomposition or change of 

 any other ; but it is perhaps scarcely desirable to add to the 

 already numerous names of stilbite-like minerals by adopting a 

 new one for this substance until additional analyses of these 

 nearly related species shall permit of their more accurate classi- 

 fication. 



