Mr Haidinger on a Remarkable Variety of' Boracite. Ill 



compound form. The small A , 



triangular planes x, x, &c. suf- y^/rT^^ 



ficiently enlarged, produce a sfs I */ h.\\\ jr 



trigonal-icositetrahedron, having LA<~ /*^[ ' N^X'"'""* 

 the general aspect of the tetrahe- V-?-A p"'r ~° j^ s \ 

 dron. This form, already de- v-^-f ,?^ \J 



scribed, and figured by Borne >l\\-/ / /? 



de Tlsle,* seems to have been ^2£££i^r 



first noticed in the varieties of boracite by Breithaupt ; + 

 Haiiy's variete plagiedre\ contains faces of it, and it is like- 

 wise indicated by Mohs.§ It is generally supposed, however, 

 to be the third variety of those forms, as described in the 

 Treatise on Mineralogy by Professor Mohs, || in which the 

 inclination of x on x, over the edge between P and s, is = 

 162° 14' 50", over the edge between n and s = 1 44° 9/ 58". 

 In this case, the edge of combination between x' and s would 

 not be parallel to the edge between s and x", which yet takes 

 place in nature ; and immediate measurement with the reflec- 

 tive goniometer likewise proves, that the two above mentioned 

 incidences are equal to each other, and very near = 152° 20 / 

 22", which is the angle of the second variety described by Mohs. 

 The situation of any single plane of the form is consequently 

 the same as that of a single plane of the tetragonal-icositetrahe- 

 dron n, first ascertained in hexahedral iron-pyrites by Haiiy,** 

 in the variety which he calls parallelique, while the situation 

 of the faces x in boracite, as he himself indicates them, would 

 coincide with the situation of the forms noted s and o in the 

 same variety of iron-pyrites. The inclination of x on an ad- 

 jacent face x, over P, is = 122° 5i° 42'. 



" Traite de Crist. PI. I. Fig. 25. 

 t Schriften der Min- Ge.i. zu Dresden, vol. i. 



% Traite de Min. 2de eil. torn- ii. p. 59. The 106th figure incorrectly 

 represents forty-eight faces, instead of twenty-four. 

 § Treatise on Mineralogy, vol. ii. p. 348. 

 || Vol. i. p. 58. •* Tald. Comp. p. 60. 



