



Theory-Dana. 



Measured. 



c : 



e 



45° 40' 



45° 30' 



b : 



m 



58° 12' 



57° 50' 



1 : 



'• '1 



47° 52' 



48° 13' 



c : 



: » 



67° 27' 



67° 18' 



c 



: t 



32° 25' 



32° 38' 



286 II'. E.Ford — Herderite Crystalsfrom Auburn^ Maine. 



one ease from the crystal constants given by Dana and in the 

 other from those given by Penfield. The variation between 

 the measured and calculated angles was, however, commonly 

 less when the Penfield angles were used. The following table 

 giving the measured and calculated values of the more trust- 

 worthy angles will illustrate these points. 



45° 25' 



57° 45|' 

 48° 22' 

 67° 20' 

 32° 40' 



Penfield has suggested that the variation observed in the 

 measured angles was due to the difference in composition, the 

 crystals measured by Dana being hydro-fluor-herderites and 

 containing several per cent of fluorine, while those measured 

 by Penfield were hydro-herderites and contained no fluorine. 

 If this idea is correct the present crystals, arguing from the 

 close relationship of their angles to the hydro-herderite of Pen- 

 field, should be near at least the hydro-herderite in composition 

 and contain only a small per cent of fluorine. This was an 

 interesting point, and in order to determine it enough of one 

 specimen was sacrificed to permit the making of an estimation 

 of the percentages of fluorine and water present. The results 

 gave, F = 6'04, H 2 = 3*62. These percentages did not agree 

 with what had been expected. They showed that the present 

 crystals were hydro-fluor-herderites and, as far as these two 

 determinations can show, that they are closely related to the 

 Stoneham material from which Dana derived his constants. 

 These results would show that if the crystal angles vary at all 

 with the variation in the percentages of the fluorine and 

 hydroxy], the change is much less than Penfield thought prob- 

 able. It is likely that the angles obtained by him from the 

 hydro-herderite of Paris, Maine, are very closely correct for all 

 varieties of the mineral. The angles derived by Dana from 

 measurement of the Stoneham crystals were not wholly to be 

 relied upon, as he says in his original description :* " Consider- 

 able difficulty was found in obtaining satisfactory fundamental 

 angles because the planes seldom afforded sharp, well- 

 defined reflections." 



Mineralogical Laboratory of the Sheffield Scientific School 

 of Yale University, 

 New Haven, Conn., June, 1911. 



* Loc. cit. 



