16 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL, 64 



Hawes, and points out that both his andHawes's minerals can be 

 interpreted as isomorphous mixtures of two compounds, namely 

 (a) RO.RA-2SiO2.2H2O, and (6) 2RO.SiO2.H2O. This is distinctly 

 not true of the present analysis in its present form. However, if the 

 ferric iron be calculated back to ferrous iron and the water below 

 110° be excluded, the formula obtained is 4Fe0.3MgO.Al203.5Si02. 

 5H2O, which may be regarded as composed of Walker's (a) x 1 with 

 RO = MgO plus (6) X 1 with RO = MgO plus (h) x 2 with RO = FeO. 

 The analysis, recalculated on this basis, is given in column 1 below, 

 while in column 2 is given the calculated composition of a mixture 

 of the above formula: 



Heated before the blowpipe in the forceps this chlorite exfoliates 

 somewhat, becomes magnetic, and finally fuses with some difficulty 

 to a black magnetic slag. In the closed tube it yields abundant 

 water having an acid reaction, indicating a small fluorine content. 

 It is quickly and completely soluble in hot 1 : 1 hydrochloric acid. 



In the same specimens there occurs a second chlorite which never 

 completely fills the cavities but merely forms a thin lining 0.5 milli- 

 meter or less in thickness in cavities containing fillings of mordenite 

 and serves as coloring matter in the surrounding andesite. It is 

 impossible to obtain enough of this material in pure form for analysis. 

 It is deep blue green in the specimen and under the microscope is 

 seen to consist of finely felted shreds and scales of a deep blue-green 

 color which, in the aggregate, do not show any noticeable pleochroism. 

 The indices of refraction are, «= 1.606, 7=1.608, 7 — a=0.002. 

 Aside from the difference in color, these chlorites differ sharply in 

 birefringence. 



In the following table the optical properties of these two chlorites 

 are repeated in comparison with those of several other occurrences 

 of similar chlorites. 



