44 R. B. Riggs — Composition of Tourmaline. 



The Orford material was received from Mr. C. H. Hitch- 

 cock, the Monroe specimen came from the National Museum 

 Collection. The schistose gangue, in which the tourmaline 

 from Orford and Monroe are imbedded is of particular inter- 

 est, and was studied microscopically as well as chemically, in 

 the hope that it might throw light on the genetic relations of 

 magnesian tourmaline. The microscopic work, which was of 

 special importance was kindly undertaken by Mr. J. S. Diller. 

 The following are the results of partial chemical analysis : — 





Orford. 



Monroe. 



Si0 2 



2*7-18 



43-30 



A1 2 3 



33-10 



27-44 



CaO 



•19 



1-96 



MgO 



28-09 



19-22 



Na 2 



undet. 



1-47 



K„0 



undet. 



•60 



Ign. 



11-75 



7-45 



100-31 101-44 



While Mr. Diller found the Orford matrix to be essentially 

 chlorite, in agreement with the results of chemical analysis, the 

 gangue rock of the Monroe tourmaline turned out to be particu- 

 larly interesting. 



Of this Mr. Diller says, " this light gray glittering rock is 

 composed chiefly of biotite, chlorite and a light colored mineral 

 which may possibly be zoisite. The biotite is very dark and 

 apparently uniaxial and negative, with all the other physical 

 properties of the species. The chlorite is much paler than the 

 biotite, and is of a greenish color. It is distinctly biaxial, with 

 a small optic angle, and positive. The relation of the chlorite 

 to the biotite is readily seen in the thin sections where it 

 evidently is derived from the latter by a process of alteration. 

 An interesting feature is that in the immediate vicinity of the 

 imbedded tourmaline the biotite is all changed to chlorite, and 

 is arranged with its foliae approximately perpendicular to the 

 cry stall ographic planes of the tourmaline, against which it im- 

 pinges. The chlorite completely envelopes the tourmaline and 

 the other portions of the hand-specimen are made up chiefly of 

 biotite and the zoisitic mineral, with a small proportion of 

 chlorite. This variation in the mineralogical composition of 

 the hand-specimen readily explains the discrepancy there at 

 first appeared to be between the results of chemical analysis and 

 my observations." That the analysis represents a portion. of 

 the rock rich in chlorite and poor in biotite, the high magnesia 

 and low alkalies plainly show. The relation existing between 

 the biotite, chlorite and tourmaline in this Monroe matrix is 

 instructive, indicating, as suggested, the transition to be from the 

 mica through the chlorite to the tourmaline. 



