3^0. 2348. MINERALS FROM TRUMBULL, CONNECTICUT— SHANNOX. 475 



1. Analysis of prochlorite from Trumbull, CaO and MnO united with FeO 

 witliout recalculation. 



2. Values to satisfy formula 2FeO. 2MsO. AI2O3. 2Si02. 2H2O. 



It is noteworthy that a large majority of the best analyses of 

 prochlorite agree quite as well with this composition as they do with 

 «ach other. In the present instance the amount of lime is unusual 

 and suggests contamination by calcite, but this was definitely not the 

 case the homogeneity and purity of the analyzed sample being thor- 

 oughly proven both chemically and optically. While there is danger 

 of overdoing the simplification of formulas, especially those of com- 

 plex minerals like the chlorites, the tendency heretofore has been 

 quite in the opposite direction, complex molecules containing hun- 

 dreds of atoms being assumed to explain analyses too often in- 

 accurate or made upon material of doubtful purity. The writer has 

 recently critically examined several species of the chlorite group 

 without finding it necessary to call upon a complicated or irrational 

 formula to explain any analysis made upon homogeneous and opti- 

 cally distinct material. Results at present are wholly tentative, but 

 the investigations will be continued as opportunity offers, and it is 

 hoped that a plausible and thorough revision of this group of min- 

 erals can be advanced when sufficient work has been completed. 



The mineral under discussion has the following properties : Color, 

 greenish black; luster, somewhat pearly; powder, pale green. Be- 

 fore the blowpipe exfoliates somewhat and fuses with difficulty to a 

 black magnetic slag. It is slightly attacked by hot nitric or hydro- 

 chloric acid. After long digestion in hot sulphuric acid colorless 

 scales remain. Yields neutral water at a high temperature in the 

 closed tube. 



Optically the mineral is biaxial, with the axial angle, 2V, ap- 

 proaching zero. Optically positive ( + ) ; acute bisectrix perpen- 

 dicular to the perfect cleavage. Under the microscope is seen to be 

 made up of plates of hexagonal outline, transparent and of a deep 

 green color. The pleochroism is distinct: 



0= bright brownish grass green. 

 /3= bright brownish grass green. 

 7= pale greenish brown. 



Refractive indices: 



a=1.621it.003. 



/3= l.618±.0O3. 



7=1.618±.003. 



a^= .005±.003. 



For comparison the similar prochlorite from the District of Colum- 

 bia described by Merrill (Cat. U.S.N.M. No. 45,875) was examined 

 ^nd found to have the following optical properties: Biaxial; 2V, 



