156 



side. This indicates pyramidal hemihedrism. I have tried to 

 produce etching figures in order to etablish this fact. But my 

 attempts have been unsuccessful. The only acid that acts upon 

 the mineral is hydrofluoric acid. But it only makes the faces 

 dull. Neither macroscopically nor under the microscope have I 

 been able to observe any distinct etching figures. 



On a few crystals from one locality the prism of the 

 second order a is alone present in the vertical zone (Fig. 6, 

 Plate VIII). The only pyramidal form that has been observed 

 on these crystals is the fundamental pyramid p. It does not 

 occur on many crystals , and its faces are always very narrow 

 (Fig. 7, Plate Vlll). On the crystals which have only the prism 

 of the second order in the vertical zone the fundamental 

 pyramid is present as small triangular truncations of the corners 

 (Fig. 6, Plate VIII). 



As regards the physical properties of the faces the following 

 facts may be stated. The basal planes are always somewhat 

 uneven and seldom bright. The unevenness is due to small 

 rounded elevations, which mostly seem to be irregular, but 

 occasionally, especially on small cube shaped crystals , assume 

 the form of very obtuse pyramids, which seem to be arranged 

 parallel to the main pyramid. All the vertical faces are highly 

 briUiant, but generally also strongly striated in vertical direc- 

 tion. The faces of the fundamental pyramid are smooth and 

 brilliant. From the above it will be understood that the 

 crystals are not suited for accurate reflection measurements. 

 The base rarely gives good or fair reflections. The vertical 

 faces give , owing to the striation , only approximate angular 

 values with one another. On the other hand the angle between 

 the pyramidal and prismatic faces can be determined with 

 desirable accuracy, and the axial ratio calculated from it may 

 therefore, be regarded as quite exact. 



