334 S. L. Pen field — Crystallization of Herder ite. 



to signify that the} 7 are in twin position. Fig. 1 represents a 

 fragment which was detached from a specimen in the Brush 

 collection. It illustrates monoclinic symmetry in the develop- 

 ment of e, q and n larger than e, q and n and also in the occur- 

 rence of the positive pyramid x between b and q and not 

 between b and q. A thin section was prepared from this crys- 

 tal parallel to b and it showed no indication of twinning. Fig. 

 8 represents the development of the forms on a crystal in the 

 Bement collection. This was so attached that the majority of 

 its faces could be observed and the monoclinic character is well 

 shown by the occurrence of d and _p in the upper front and 

 lower hind segments only. 



Fig. 9 represents a crystal in the Bement collection which is 

 reported as being the largest herderite ever found at Stoneham 

 and has already been described in this Journal by Mr. Hidden.* 

 Its actual dimensions parallel to the crystallographic axes are 

 a 23 mm , £~25 mm and 6 18 mm . The crystal was attached at one 

 extremity of the a axis and has been drawn with 010 in front. 

 It is twinned, and what is undoubtedly the line of twinning 

 may be distinctly traced across the m and x faces, as indicated 

 by the dotted line in the figure. That portion of x above the 

 twinning line is quite perfect, and with reference to the upper 

 half of the crystal, it is situated in its normal position for 

 monoclinic symmetry as a positive hemi-pyramid. The lower 

 portion, however, is very uneven and vicinal and occupies the 

 position of a negative hemi-pyramid, but this has not been 

 included in the list of forms, as this is the only occurrence of 

 it that has been observed and it is not a perfect face but rather 

 an attempt to complete the x face in the upper portion of the 

 twin. 



As has already been stated the material from Stoneham is a 

 hydro-fiuor herderite with OH : F = about 3 : 2 and it presents 

 an excellent, opportunity for comparison with the hydro herd- 

 erite from Paris, and of observing the variations in the phys- 

 ical properties produced by the partial substitution of fluorine 

 for hydroxyl. Unfortunately the crystal faces have such a 

 strong tendency to vicinal development that it is difficult to 

 obtain reliable measurements. Owing to the absence of visible 

 reentrant angles on twin crystals the inclination /? must be 

 very near 90° but it could not be accurately measured. On 

 two crystals which were selected on account of the perfection 

 of their faces the following measurements were made, of 

 which those under I are especially good : 



* Loc. cit. 



