478 Mr, L. Fletcher's Cryatallographic Note*, 



eobaltine, to which it was very similar in habit, though differ- 

 ent in colour. Closer examination, however, made known the 

 fact that the hemihedrally-developedfaces belonged, not to the 

 pentagonal dodecahedron 7r(2 1 0), so common in both cobal- 

 tine and iron pyrites, but to the much rarer form 7r(310). 

 In this Collection there are fourteen more or less perfect crys- 

 tals, presenting respectively the following development : — 



No. 1 shows only the octahedron o(l 1 1) modified by faces 

 of the icositetrahedron n(2 1 1), as shown in fig. 1 (PI. X.). 



Nos. 2, 3, 4 show also the faces of the dodecahedron d{l 1 0), 

 and are represented in fig. 2 — each of these four crystals being 

 implanted upon a crystal of cobaltite in the way described by 

 Scheerer and vom Bath. On Nos. 5 and 6 the same forms 

 recur; but some of the quoins are truncated by small planes 

 of the cube a(l 0), probably due to cleavage. 



As in all the above forms the poles of the faces lie in dode- 

 cahedral planes, the abeyance of symmetry of the latter would 

 have no effect in reducing the number of faces; in other words, 

 so long as only these forms are present it is impossible to distin- 

 guish crystallographically whether the structure is character- 

 ized byholohedral symmetry or by diplohedral hemisymmetry. 



Nos. 7 to 11 present faces of the tetrakishexahedron/(3 1 0); 

 and it now becomes possible to determine whether the internal 

 structure as shown by the external form is to be regarded as 

 holohedral or hemihedral. As a matter of fact, in each of these 

 crystals the faces of only one semiform ir (3 1 0) are found 

 to be present, the number varying with the more or less frac- 

 tured state of the crystal. This combination is represented 

 in fig. 3, from which, for the sake of simplicity, the small 

 faces of the cube have been omitted. No. 7 presents all the 

 twelve faces required by perfect hemisymmetry; No. 8 shows 

 eight faces, No. 9 shows five, while on Nos. 10 and 11 only 

 two are present; but on none of these crystals can any faces 

 of the complementary semiform it (1 3 0) be distinguished. 



Crystal No. 12 is five eighths of an inch (=1*6 centim.) 

 long, and projects from a matrix of quartz and mica. It is par- 

 ticularly interesting as showing the hemihedral development, 

 not only of the above tetrakishexahedron, but also of an 

 hexakisoctahedron having its planes in the edge-zones of the 

 dodecahedron. The angle made by the faces of this new form 

 with the adjacent faces of the form (2 1 1) was measured by 

 help of the telescopic images to be 10° 48' in one case and 

 11° 2' in another; there is no doubt, then, that the faces 

 belong to the semiform it (3 1 2), for which the corresponding- 

 calculated angle is 10° 54'. Fig. 4 represents the actual de- 

 velopment of the various faces on that part of the crystal 



