Hinsdalite, a new mineral. 253 



colorless, with a greenish cast, but much of the mineral is dark 

 gray from minute inclusions. 



A microscopic study of thin sections revealed the fact that 

 the larger crystals, at least, are strongly zoned. This zonal 

 structure is easily seen in ordinary light, or even in the hand 

 specimens. Some of the zones are much more subject to alter- 

 ation than others. Between crossed nicols the different zones 

 show slightly different interference colors. 



On account of the zonal growth it is not possible to obtain 

 accurate and consistent optical data. Cleavage pieces of the 

 mineral are not isotropic, but are nearly normal to a positive, 

 acute bisectrix. The values of 2E vary from nearly 0° to 40°. 

 Dispersion of the optic axes was not perceptible. In many 

 cases the core of the larger crystals is nearly uniaxial, and the 

 most common value of 2E for the outer zone is 32°. The vari- 

 ation in the value of 2E is probably largely due to the zonal 

 growths, though in some cases it is clearly due to overlapping 

 of twin lamellae. Basal sections of some of the crystals are 

 divided into six radial segments and the plane of the optic axes 

 for each segment is normal to the hexagonal prism edge and to 

 the striations. 



These relations are shown in fig. 1, where the diagrammatic 

 part shows the optical relations and the photograph shows a 

 basal section (under crossed nicols) of an incomplete hinsdalite 

 crystal. 



Accurate measurement of the angle between the segments 

 was not feasible, though it approximates closely to 60°. Similar 

 optical anomalies have been described for hamlinite* and for 

 jarositef, both members of the same series of minerals to which 

 hinsdalite belongs (see paper in the number following). 



The indices of refraction for hinsdalite were determined by 

 the oil immersion method. The values of a and /3 are nearly 

 the same and vary from 1/66 to 1*68, with an average value of 

 1-67 ; the value of 7 varies from 1-678 to 1-700. The bire- 

 fringence was measured by comparing the interference color of 

 sections parallel to the plane of the optic axes with that of the 

 same section of quartz. For the greater part of the hinsdalite 

 crystal the value of - 019 was obtained. Some of the narrow 

 zones gave a value as low as - 016. Therefore the indices of 

 refraction for hinsdalite may be taken as: a = 1-670, /3 =1-671, 

 7= 1-689. 



* Bowman, H. L. : On Hamlinite from the Binnentkal, Switzerland. Min- 

 eral. Mag., vol. xiv, p. 389, 1907. 



f Slavik, Franz: Mineralogische Notizen. Zeitschr. Kryst., vol. xxxix, 

 p. 297, 1904. 



Am. Jour. Sci. — Fourth Series, Vol. XXXII, No. 190. — October, 1911. 

 19 



