298 Formation of Compound or Twin Crystals. 



union of B and C with A was effected at the same instant, and con- 

 sequently they have the same situation on A ; but here the addition 

 of C was subsequent to the union of A and B, and from this has 

 arisen its equal inclination to the other molecules. Calculation will 

 here also show that the prism from such a nucleus would have the 

 angles above stated. 



Crystals of Arragonite often present a similar composition. 



Fig. 6', is a representation of a six-sided prism of Witherite, de- 

 pending on the compound nucleus in Fig. 3. The Hexagon in 

 this figure has the same angles as the horizontal section of this 

 prism. In the nucleus, the molecules A and B, and C and D were 

 first united adaxally as in Fig. 11, PI- !• These compound mole- 

 cules were then brought in contact by the attraction in the adjacent 

 poles, (of opposite polarity,) whose axes are consequently in the 

 same straight line. Of the angles of this prism, a, c, b and d will be 

 the obtuse primary angles of the primitive form, and m, n, equal to 

 twice the acute. 



When the lateral edges of the component prisms are truncated, 

 there results the stellated form in Fig. 4, which is a horizontal sec- 

 tion of the cruciform crystal in Fig. 5. This is a very common form 

 both of Arragonite and White Lead ore. 



A stellated crystal of six individuals radiating from a centre, is rep- 

 resented in Fig. 8, of which a horizontal section is given in Fig. 4, 

 together with the prismatic form that would result from the included 

 nucleus. It will be observed that composition has taken place at all 

 the lateral poles of the central molecule A, or parallel to all the fa- 

 ces of the circumscribed rhomb, instead o^ two as in Fig 1. Thus 

 B and C, D and E are united to A. The prism a oh f c o eg pro- 

 ceeding from this nucleus, has a reentering angle at o, correspond- 

 ing to the same reentering angle in the nucleus. 



The truncation of the lateral edges of the four crystals B, C, D, 

 E, would give rise only to a cruciform crystal. But the action of 

 the central molecule A, together with that of the poles m, n, would 

 cause an addition of particles parallel to A and thus give rise to the 

 two other rays. The commencement of these rays will be by an 

 interaxal composition with the particles B, C and D, E, the added 

 particle having two of its poles near m and n. The rays it will be 

 observed, have the direction before pointed out, that is, are in the 

 line of the short diagonal of the rhomb, or the minor axis of the el- 

 lipse. This form occurs frequently in Arragonite and Carbonate of 

 Lead. 



