ON Tin: ^iEIONITE. |(^5 



I. Feldspar. 



The primitive form or' the feldspar, according to Haiiy*, Primitive form 

 is an oblique angled parallelopipedou, in which the angle of "^ ^'^'^'^^P^"'' 

 incidence between M and P is of 90°, that between M and 

 T of 120°, and that between T and P of 111° 23' 17" See 

 in fig. 1 this solid represented in the position given it by Mr. 

 Muhs himself, as being favourable to the comparison he 

 inukc'S of the two substances. Mr. Ilauy observes in his 

 treatise, it is true, that t)ie sections parallel to M and P are 

 very clear, and very easy to obtain ; while that parallel to T 

 ■simply shows itself by a changeableness of colour in a strong- 

 light. Since the publication ^j^ this treatise however, this 

 gentleman has. obtained from the feldspar, by mechanical 

 division, nuclei presenting the joint parallel to T in a very 

 clear and decided manner, which he has publicly shown in 

 his late courses of lecture;;, and some of which he has dis- 

 tributed among his auditors. 



The primitive form of the Feldspar once thoroughly as- Can th? forms 

 certained, it remains to be known, whether, setting out from of the meionite 



1 • 1 1 .L • 1 ...1 1 CI 1 be produced bv 



this nucleus, we can obtam by the laws oi decrement the any decremeat 

 forms of the meionite. But the mere inspection of the °* ^'^**' 

 crystals shows at once the impossibility of this. In fact, the ^°* 

 meionite has the four faces of its summit equally inclined 

 to each other and to the lateral faces. Now this symmetry 

 is incompatible with any primitive form but a prism with 

 square bases, as in the mesotype, or a rectangular octaedron, 

 as in the zircon ; both which species exhibit forms analogous 

 to thos.'^ of meionite, but with different incidences. It is al- 

 together the reverse with the forms of the feldspar, which 

 bear in some sort the impression of the irregularity of their 

 primitive form in the want of symmetry of the fixces arising 

 on parts similarly situate. The following details appeared to 

 ine necessary, to place this proof in a clearer and stronger 

 lij^ht. 



Fig. 3 represents one of the forms of feldspar, in which Two crystals 

 the faces M,P, T of figure 1 are preserved, and the face O b!e°\o ihe^sup- 

 F . , position takan. 



results from the decrement -£ according to the position the 



O 2 nucleus 



• Traite de Min^ralogie, torn. ?, p. 591. 



