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IV. On the Crystalline Form of Faroelite. 

 By Dr. Heddle*.- 



WHEN in Faroe, I obtained at the cave in Naalsoe globules 

 of Faroelite, composed of crystals large enough to aiford 

 satisfactory measurements with the reflecting goniometer. They 

 are as follows : — 



Pa 

 PA 



a b 

 (cleavage) — 



Mfl 116 20 



90 00 

 90 00 

 90 00 



Form, abp. 



Cleavage parallel to a highly perfect ; parallel to b perfect ; 

 M imperfect. Lustre, a pearly ; b P and M vitreous. 



The Skye crystals are similar ; but some of the Irish speci- 

 mens seem to have planes truncating the edge P b or the solid 

 angle aVb; these crystals, hoAvever, are too rough and too small 

 for measurement. 



I have already shown in this Journal (January 1857) that 

 the chemical constitution of this species is quite distinct from 

 that of mesolite, with which it used to be confounded ; and the 

 above measurements definitely set the matter at rest, as the 

 mineral is prismatic instead of obhque, being near Thomsonite, 

 and isomorphous with stilbite. 



It stands, indeed, intermediate, and connects the chain between 

 Thomsonite and mesolite. 



Thomsonite {normal) . Ca^ Si + 3A1 Si + 7H. 



Soda Thomsonite . . (Na, Ca^) Si + 3A1 Si + 7H. 



Faroelite (Na, Ca^) Si^ + 3A1 Si + 8H. 



Mesolite (Na, Ca^) S> + SAlSi + 8H. 



With Scolezite its connexion may be seen by adding — 



Scolczite 3(CaSi)+3AlSi=9H. 



With natrolite by adding — 



Fargite (Na^ Ca) Si^ + 3A1 Si + 7H. 



Natrolite 3(Na Si) + 3^181 + 6H. 



N.B. The crystals from which the figure was drawn were 

 examined chemically, to determine that they were not stilbite. 

 * Communicnted by the Author. 



