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 VI. On Mesolite and Faroelite {Mesole). By Dr. Heddle*. 



IN a late Number of the Magazine I submitted analyses of 

 natrolite, so named from the large quantity of soda it con- 

 tains; its formula is NaO, SiO^ + AFO^ Si03 + 2HO. The 

 mineral which stands next to this in the system is scolezite, the 

 formula of which is CaO, Si03 + A120^ SiO^ + SHO ; differing 

 from natrolite in containing lime instead of soda, and in having 

 an additional atom of water. It is thus rational to presume that 

 the most common impurity in either of these substances would 

 consist of a greater or less amount of the bases which constitute 

 their specific distinction; that natrolites would be found con- 

 taining some lime, and that scolezites would occasionally be 

 Contaminated with soda. Published analyses show that this is 

 the case ; and I have lately pointed out, that, in the case of "ga- 

 lactite," the occurrence of an impurity of this nature had for years 

 given a distinct name to what is merely an instance of chemical 

 replacement. So long as this substitution of one substance or 

 element for another occurs in varying proportion, or in such as 

 cannot be expressed by a rational formula, the substance in 

 question is to be considered as merely a chemically impure (I 

 use chemically here to distinguish from mechanically impure) 

 specimen of that mineral which in composition it most resembles ; 

 when, however, the replacement occurs in definite proportions, 

 then a distinct mineral is before us, with as good claims to be 

 considered a species as either of the originals, the result of whose 

 combination we may theoretically consider it. Thus, in my five 

 analyses of galactite, we had the lime of scolezite occurring in 

 the soda mineral in the per-centage proportions of 'IS, '83, 

 •93, 2-63, 4-312, no one of which proportions! agrees with 



* Comimmicated by the Author. 



t Except, perhaps, the last; the specimen was from Glen-fai'g. The 

 analysis stated in full was, — 



Sihca .... 47-84 



Alumina . . . 27-112 



Lime .... 4-312 



Soda .... 10-304 



Water .... 10-24 



These proportions point to the formula '^■^ ^^° 



(2NaO, CaO)3Si03+3(APO', Si03) + 7IIO, 



But the above formula may be written / 30686 

 2(NaO, SiO^-l-APO^ Si03+2HO)+(CaO, SiOHAl^Qs, SiO^+SHO) 

 2 atoms natrolite -|- 1 atom scolezite 



the very reverse, in fact, of mesolite. This compound may be called Fargite. 



