542 R. P. D. Graham — Optical Properties of Hastingsite. 



But there are always a few fragments lying on a plane 

 which is approximately normal to the acute bisectrix and when 

 these are sufficiently thin they exhibit a fairly well-denned 

 optical figure. As had been previously noted, the central part 

 of the interference figure, even when in the diagonal position, 

 is not well illuminated, owing to the deep color and the weak 

 double refraction of the mineral. The quasi-uniaxial figure is 

 colored red in one pair of opposite quadrants, transverse to the 

 cleavage cracks, and bluish green in. the other, and it is usually 

 difficult to decide as to which of these directions is the line 

 joining the optic axes. If the axial plane lies in the plane of 

 symmetry, as is ^usual in hornblende, then the angle for red is 

 greater than that for blue, or p > v. But when exceedingly 

 thin and less highly colored chips are examined, it is found 

 that the hyperbolas open out across and not along the plane of 

 symmetry. They are colored red on their concave side, and 

 although the brushes are thick and frayed, a certain amount of 

 bluish green light gets through in the narrow space which 

 separates them, and the same color tints the rest of the figure. 

 It was at first thought that this effect might be only an illu- 

 sion and due to the fact that more light is transmitted near the 

 red portions of the figure than elsewhere, thus causing an 

 apparent opening of the hyperbolas in this direction. But the 

 phenomenon was observed in many cases so clearly as to admit 

 of no doubt in the writer's mind that in hastingsite the axial 

 plane, for green light at least, lies at right angles to the plane 

 of symmetry of the mineral ; the axial angle for reel light is 

 less, and there may even be a crossing of the optic axial plane 

 for these colors, since the interference figure observed in yel- 

 low light approaches more nearly to the uniaxial cross, 

 although it is very ill-defined owing to the poor illumination of 

 the field. The birefringence is weak and negative. Although 

 the pleochroism of sections cut in this direction is compara- 

 tively slight, it is in the same sense as that noted above for 

 those parallel to the prism. 



Considered with reference to the crystallographic axes, the 

 pleochroism is like that usually met with in amphiboles ; but 

 since in the case of hastingsite the plane of the optic axes lies 

 across the plane of symmetry instead of along it, we have 

 b>c>a, b and c being nearly equal. 



It is impossible to make any accurate measurement of the 

 optic axial angle, but it is evidently quite small. It was 

 thought that it might be useful to make a rough estimate of 

 its value by comparison with some other mineral of small 

 angle, and biotite was selected for this purpose. A cleavage 

 flake, in which the hyperbolas separated to about the same 

 extent, so far as could be judged by the eye, as in the case of 



