290 Formation of Compound or Twin Crystals. 



be in some cases of hemihedral modification. When these angles 

 are all oblique and unequal, as in the Oblique Rhomboidal Prisms^ 

 this kind of composition may produce a different twin crystal, on 

 each of the primary faces. 



Instances of compound forms of this kind, are of very frequent 

 occurrence in Arragonite and Carbonate of Lead or White Lead 

 ore, which sometimes present stellated forms, owing to a repetition 

 of the composition. Fig. 13, represents a crystal of Arragonite thus 

 compounded parallel to the face P. A vertical profile of the same 

 is exhibited in Fig. 13 a, in which it is more distinctly seen that the 

 composition takes place parallel to a lateral face. The peculiarity 

 of this form is owing in part to the truncation of the acute lateral 

 edges by the planes e, e. Fig. 13 6, shows the relative situation of 

 the molecule and the lateral planes e, e. Other minerals in which 

 these forms have been observed, are Periclin, White Iron Pyrites, 

 Albite, Epidote, Gypsum, Feldspar, &c. For other figures of these 

 forms, reference may be had to most works on Mineralogy. A 

 treatise by C. U. Shepard,* is quite full in them, as also in the fig- 

 ures of crystals generally. A valuable article on compound crystals 

 by Haidinger in Brewster's Edinburgh Journal, is accompanied with 

 numerous figures. Naumann's treatise on Crystallography,! con- 

 tains figures of upwards of one hundred and fifty different forms of 

 Twin Crystals. 



In the formation of other crystals, which are exceptions to the 

 general principle, two molecules unite in points of equilibrium of at- 

 traction between two poles in some instances, and in others in the same 

 point between three poles. In the first case the situation of the mo- 

 lecules is similar to that given in Fig. 12, where they are retained 

 in combination by the action of two north poles of one, on the two 

 south poles of the other. It is apparent that this is an instance of a 

 composition parallel to an edge, as the edges in the Primary forms,, 

 lie opposite the points of equilibrium of attraction between two poles. 



* Treatise on Mineralogy, consisting of descriptions of the species and tables 

 iUustrative of their natural and Chemical affinities. By Charles U. Shepard, 

 Lect. on Nat. Hist, in Yale College, &c. 2 vols., 12mo., with 500 wood cuts. New 

 Haven, 1835. 



t Lehrbuch der reinen und angewandten Krystallographie, von Dr. Carl Frie- 

 drich Naumann, 2 vols., 8vo., with 39 copper plate engravings, containing 900 

 figures af Crystals. Leipzig, 1830. 



