THOMSONITE OF TYPE I 465 



Figure 1 of plate 43 gives an excellent idea of this habit. The glob- 

 ular appearance is very characteristic of many of the specimens. In the 

 lower part of the figure may be seen two or three of the spherical aggre- 

 gates broken open so as to show the radiated structure and the satin-like 

 luster. In this specimen the surface of the cavity was first coated by 

 a continuous thin layer of chabazite, which does not show in the 

 figure. This, in turn, was completely covered by the thomsonite. A 

 later generation of thomsonite, to be described under type Ilia, forms 

 rough surfaced globular aggregates grown on the surface of the first 

 generation. 



In their description of the individual minute leaves that make up 

 these radiate aggregates, Cross and Hillebrand refer to the macropinacoid 

 as the dominant form with subordinate prism and brachypinacoid, the 

 terminal face being the basal plane. As the prism is nearly at right 

 angles and the leaves examined measure only about 0.01 millimeter in 

 thickness, this determination is naturally only a surmise, based on the 

 generally accepted forms given in the standard mineralogies. In spite 

 of the minuteness of the constituent leaves, it is quite possible, however, 

 by optical investigations to show that the brachypinacoid rather than 

 the macropinacoid is the dominant form. This determination is based 

 on the generally accepted fact that the best cleavage is parallel to the 

 brachypinacoid, and the plane of the optical axes is parallel to the basal 

 pinacoid, with the acute positive bisectrix parallel to the macro-axis. 

 According to these accepted properties of thomsonite, one would expect 

 to observe a positive bisectrix, with the axial plane at right angles to the 

 vertical crystal axis on the best cleavage planes. These observations 

 have been verified in numerous cases on individual leaves and on care- 

 fully selected cleavage pieces, as well as on random cleavages produced 

 by pulverization. The mineral cleaves very easily parallel to the leaves 

 and only indifferently at right angles to the same. This determination 

 of the position of the positive acute bisectrix was also supported by the 

 behavior of the same material in parallel polarized light. Individual 

 leaves and cleavage pieces invariably show the axis of least elasticity 

 parallel to the vertical axis. The extinction is always parallel. It 

 should be stated, however, that when the cleavage pieces are not selected, 

 but are obtained by pulverization and examined at random, occasion- 

 ally one may lie on the poorer macropinacoidal cleavage plane. In this 

 case the vertical axis is the axis of greatest elasticity. 'It may be well 

 to state here in anticipation that in case of all the other types of thom- 

 sonite, wherever the forms are tabular, the brachypinacoid may be shown 

 in the same way to be the dominant form. 



