[graham] a new ZEOLITIC MINERAL 187 



Radiated groups of ferrierite crystals are sometimes seen on the outer 

 surface of such specimens, embedded in the chalcedony. 



Crystallography and General Physical Properties 



Individual crystals of ferrierite have the form of very thin blades, 

 which are rectangular in outline, but isolated crystals of this type 

 are seldom seen. In the specimens, these blades are stacked upon one 

 another in nearly parallel position, after the manner of the leaves in 

 a closed fan; a large number of such piles of slightly divergent blades 

 radiate from a common centre, and give rise to the spherical aggre- 

 gates. The true form of the latter, and the manner in which they 

 are built up, becomes very evident when the spheres have a covering 

 of calcite, and this is removed by solution in acid. It is then seen 

 that the ferrierite does not, as a rule, form a continuous, solid sphere, 

 but that spaces have been left between the variously inclined piles 

 of blades, and have been filled by the subsequent deposition of calcite. 

 The structure is no doubt similar when chalcedony forms the enclosing 

 material. Here, cross sections of the spheres usually appear as com- 

 plete circles of radiated ferrierite blades, but on closer examination 

 white streaks of the silica may usually be seen between the blades, 

 especially towards the circumference of the circle. 



The optical study of the mineral proves it to be orthorhombic, 

 and if the crystals are oriented as tabular parallel to a(lOO), with 

 elongation along the c-axis, the forms exhibited are the pinacoids, 

 a (100) and /; (010), and the macro-prism or dome J(lOl). The angle 

 100 : 010 is 90° 00'. The dome faces form the terminal planes of the 

 blades, but, although they are often of fair size, they are always 

 imperfect, and yield multiple reflections as a result of the parallel 

 growth already referred to. Only two or three very thin blades were 

 found, on which the dome face yielded a single, but faint, image, 

 and the mean of the measurements made on these gave the angle 

 ad = 67° 47', from which dd' = 44° 26'. No twin crystals were observed, 

 nor did the optical examination reveal any evidence of twinning. 



The mineral has a perfect cleavage parallel to a(lOO), and on 

 this face the lustre is pearly; the lustre on 6(010) is bright vitreous, 

 but on the dome faces it is duller, doubtless owing to their imperfect 

 nature. Individual blades or thin cleavage plates are perfectly 

 colourless and transparent, but in the crystal aggregates the mineral 

 appears white; in some weathered specimens, it is stained reddish 

 by iron oxide. The outer ends of the blades, near their contact with 

 the enclosing chalcedony, are quite generally translucent and milky, 

 while thin white streaks of chalcedony penetrate for a short distance 

 between the blades; also, a narrow zone of the chalcedony, immed- 



