tS. L. Penfield — Crystallization of Herder ite. 335 





Measured 



Calculated, 



Calculated for 





I. 



II. 



Dana. 



hydro-herderite. 



m/^m, 



110^110 = 



63° 57' 



63° 39' 



64° 29' 



n ,\ n, 



331,^331 = 5«° 19' 



58 21 



58 17£ 



58 59 



q /\ q, 



332^332 = 



47 58 



47 52 



48 27 



s ^ s, 



061^061 = 137 12 







137 2 



137 23^- 



It will be observed in the above that the measurements agree 

 very well with the angles calculated by Dana but vary con- 

 siderably from those found on hydro-herderite. The axial 

 ratio established by Dana is therefore retained, and is repeated 

 below along with the author's ratio for hydro-herderite : 



Hydro-fluor-herderite a :T: ~c = '0'62060 : 1 : 0-42345 (3 = nearly 90° 

 Hydro-herderite " " = 0-63075 : 1 : 0-42742 (3 = 89° 54' 



The partial substitution of fluorine for hydroxyl has had, 

 therefore, an appreciable effect upon the axial ratio, shown 

 especially by a perceptible shortening of the a axis. It may 

 also be pointed out that the specific gravity of the Stoneham 

 mineral, 3'006-3*012, is greater than that of the hydro-herd- 

 erite from Paris and Hebron, 2*952-2*975, which might be 

 expected, as fluorine with an atomic weight of 19 is heavier 

 than hydroxyl with a molecular weight of 17. 



Optical properties. — Several clinopinacoid sections that were 

 prepared showed an inclined extinction of about +2° when 

 measured with yellow light. Sections from twin crystals like 

 figs. 5 or 6 revealed the twinning beautifully in polarized light. 

 Fig. 14 represents an ideal section, the opposite portions I and 

 I as also II and II extinguishing simultaneously. Fig. 15 rep- 

 resents the actual disposition of the Parts I and II in a section 

 from a crystal like fig. 6, the arrows indicating the directions 

 of extinction. 



The mean index of refraction was determined by means of 

 a prism cut with its edge parallel to the ortho axis and the 

 divergence of the optical axes by a plate cut normal to the 

 acute bisectrix. The results, all determined in yellow light, 

 Na flame, are as follows : 



Hydro-herderite. 

 From prism a = 22° 55' 6 = 14° 24' hence j3 = 1 -612 (3 =1-630 

 2Ea= 128° 25' hence 2Va = 67 56' 2V=7l°59' 



2Ha= 66 hence 2Va — 68 7 71 59 



The double refraction is negative. The dispersion is p > v 

 and is distinctly inclined. The indices of refraction have 

 been determined by Bertrand* as follows : 



a =1-621 /? = 1-612 ?* ='1*592 



*Bull. Soc. Min. de France, ix, p. 142, 1886. 



