Dr. Blum on Pseudomorphous Minerals. 71 • 



tion ; but Dr. Blum is disposed to consider carbonic acid as usu- 

 ally present when it is in progress, and alludes to instances of 

 regions of carbonated waters where the process goes on with un- 

 usual rapidity. 



The formula for 3 atoms of feldspar is Al3 Si'+K^ Si^, or 



summing up, AP K'' Si'=. Remove now K^ Si% and it leaves 



AP Si*, which is the composition of kaolin, except that 6H is 

 added. 



Kaoli7i with the form of Porcelain Spar. — At Obernzell, ac- 

 cording to Fuchs, kaolin is largely derived from porcelain spar. 

 This mineral occurs in granite, along with opal and semiopal, in 

 incrusting and nodular masses. This opal is believed to have 

 proceeded from the silica liberated during the process. From 



3[4CaSi + 4AISi+NaCl], 12Ca, 8Si, 3NaCl pass off, probably 



by the action of carbonated waters, and 24H is added ; making 



4[Al3 Si*-F6H], or 4 atoms of kaolin. 



Kaolin with the form of Leucite. — This change consists sim- 

 ply in the removal of the potash, K% from K^' Si^ + 3A1 Si^ 



(leucite) and the addition of 6H, making thus Al^Si^ + GH. 



Mica with the form of Andalusite. — Crystals of Andahisite 

 have been observed changed wholly to a light colored mica, and 

 others in which this change was but partially complete, the ends 

 being mica and the middle Andalusite. In the change, silica and 

 potash are added. 



Mica with the form of Scapolite. — Dr. Blum describes a crys- 

 tal of scapolite altered to mica, from Arendal, which is three 

 inches long, and consists of an aggregate of mica scales. Along 

 with the altered scapolite there is a soft talc-like mass, which 

 also appears to have been produced by the alteration of this mine- 

 ral. To form the mica Dr. Blum supposes 4[Ca3 Si + 3AlSi] 

 (4 of scapolite) to have lost Si, 12Ca, and received 3k, whence 



would proceed 3[k Si4-4(A1, Pe)Si], or 3 atoms of mica. Yet 

 as the exact composition of the particular mica is not known, 

 there is some doubt of the correctness of this view. 



Mica with the form of Tou7'mali?ie. — Crystals of tourmaline 

 consisting of mica or with mica terminations have been observed 



