54 S. L. Penfield — Bertrandite from Colorado. 







Des Cloizeaux. 



Scharizer. 



Calculated. 



b ^ m, 



K 



010 ~ 110 





100^110, 



100 „ 110. 



61° 13' 10" 



60 c 12' 40" 



60° 13' 



a aJI, 



100^130 



59 c 57' 







59° 47' 



b aZ, 



010^130 



30° 17£' 







30° 13' 



2^2, 



130 ^ 130 



129° 30' (twin) 







129 c 34' 



a ^ a. 



100^100 



60 c 40' (twin) 







60° 24' 



c ^ e', 



001 ~031 





001^301, 



119° 15' 10" 



119 c 15' 



& . e', 



010^031 





100 a 301, 



28° 44' 20" 



29° 15' 



m ^ e, 



110 ^ 031 





110*301, 



64° 34' 20" 



64° 19' 



m a. e, 







110*301, 



64° 27' 40" 



" " 



m a. r/, 



110 ^ 021 





110*201, 



110 * ,201, 



67 c 38' 30" 

 67° 37' 30" 



67° 39' 



e a 7j. 



031^021 





301*201. 



10 c 19' 



10° 47' 



The above measured angles agree very well with the calcu- 

 lated values, and where the difference is large the reason may 

 be found in the uncertainty of the measurements made on 

 so small crystals. Scharizer's measurements agree about as 

 well with these orthorhombic values as with his own calculated 

 values for monoclinic axes. 



I cannot give a reason for the hemimorphic development of 

 the basal plane. If Scharizer is correct in assuming that the 

 crystals are monoclinic with the brachy-axis of Bertrand as the 

 ortho-axis, such a development might result from twinning 

 about an orthopinacoid, one basal plane being converted into 

 a curved surface by oscillations with hemi-orthodomes, sym- 

 metrically situated on either side of the twinning plane. This 

 would require for /9=90° 28' 34" (Scharizer's value for the 

 inclination of the a and c axes) a salient angle along the 

 twinning line on the base of 180° 57' which could not be de- 

 tected. A section across the crystals, parallel to Scharizer's 

 clino-pinacoid, should also show an inclined extinction which 

 would be especially marked along the twinning limit ; a sec- 

 tion thus prepared shows perfectly normal orthorhombic sym- 

 metry in polarized light. 



The optical properties point most decidedly to orthorhombic 

 symmetry. The obtuse bisectrix is normal to the basal plane, 

 the plane of the optical axes is the brachypinacoid. The 

 divergence of the optical axes measured with a large Fuess 

 apparatus in the Thoulet solution (?&= 1*6503 for yellow, ~Na, 

 flame), is 



2H=101° 10' for yellow. 



Using Bertrand' s mean index of refraction /9=l-569 we get 



2V=108° 42' for yellow. 



Bertrand determined 2V=105° 8', and Scharizer 2V = 108° 

 31/. The dispersion about the obtuse bisectrix is marked p> v 

 and therefore about the acute bisectrix p <v. A section j>ar- 

 allel to the macropinacoid showed the acute bisectrix in the 



