336 /S. L. Penfield — Crystallization of Herderite. 



The divergence of the optical axes has been measured by Des 

 Cloizeaux.* His optical preparation was undoubtedly cut 

 from a twin crystal, for he states that on both sides of the 

 bisectrix two sets of rings were visible, which may readily be 

 understood by reference to fig. 14, as a section parallel to the 

 ortho-pinacoid would be approximately normal to two bisec- 

 trices. He gives 2E« for the inner hyperbolae 121° 22' and 

 for the outer 130° 2'. The true value should be about mid- 

 way between these. From the study of the optical proper- 

 ties we learn, therefore, that, by the partial substitution of 

 fluorine for hydroxyl, the position of the axes of elasticity has 

 not materially changed but the mean index of refraction /? and 

 the divergence of the optical axes have decreased. 



Herderite from Auburn, Maine. 



The material from this locality as shown by the closed tube 

 reaction is a hydro-fluor-herderite, and as far as known to the 

 author is represented by a single specimen in the Bemsnt col- 

 lection and two crystals belonging to Mr. T. F. Lamb of 

 Portland, Maine. On the specimen in the Bement collection 

 there is one crystal which is almost colorless and transparent 

 and has the habit shown in fig. 10. It is attached to albite 

 and a large imperfect herderite and measures parallel to the 

 axial directions a 8 mm , h 8 mm , and c ll mm . It is twinned and 

 shows a reentrant angle formed by the faces of the dome 

 b, 301. This form is strongly vicinal in its development, and 

 was not identified by measurement but by its position in the 

 zones v, m. Some of the large faces are dull and not adapted 

 for measurement but the forms m, n and q, except for a slight 

 striation parallel to their mutual intersections, are remarkably 

 perfect and yielded the following measurements : 



Stoneham 













herderite. 



Hydro-herderite. 



m /\ m , 



upper 



crystal 



63 c 



53' 



'63° 39' 



64° 29' 



m /\ m , 



lower 



crystal 



63 



52 



a » 



tc a 



n y\ n, 



upper 



crystal 



58 



31 



58 17| 



58 59 



n ^ n, 



lower 



crystal 



58 



30 



a tc 



a a 



9^9, 







47 



54 



47 52 



48 22 



m,\ n, 







22 



22 



22 33 



22 35 



niy\ q, 







39 



36| 



39 42 



39 39 



From the values of m^m and n* n, which are undoubtedly 

 very accurate, it may be assumed that this variety contains less 

 fluorine than the Stoneham mineral as the values are slightly- 

 greater and thus approach the hydro-herderite. The crystals 

 from the collection of Mr. Lamb are in habit similar to the one 



*Bull. Soc. Min. de France, vii, p. 132, 1884. 



