38(> On certain large Crystals of Knstatite* 



Efere also the face k is a little dislocated : 



h : m=lQ3 6 101 i 



k : m'= 99 45 — 



It is possible, even easy, to reduce the above-mentioned 

 crystals to the form of enstatite, as the perturbations which 

 produce a monoclinic appearance are only 1 or a few degrees. 

 On other crystals it is impossible without making too arbitrary 

 assumptions, because the differences between the homologous 

 edges are as great as 7 degrees, even more in some cases. 

 The two large crystals of the Bonn Museum have such a habit. 

 One of them (see fig. 7) allows pretty exact measurements, 

 If we had had only this single one, we should not have dared 

 to assign it to the rhombic system of enstatite ; and yet even 

 here no doubt remains that the irregularity is due to perturba- 

 tions only. We acquire this conviction, as well by comparison 

 with the more regular specimens as by an exact study of the 

 perturbed crystal itself if we try to reduce it to the monoclinic 

 system. We get in this case no simple symbols, and we are 

 obliged to take different axial elements for nearly every crystal. 

 The best-formed specimen of the Bonn collection gave the fol- 

 lowing angles (fig. 7) : — 



m: m' (over 5) = 89° 40', 



m: % = 109°, 



ml: %=102°. 



If we regard m as prism c©P, % as klinodome (P oo), we 

 obtain the following elements : — 



a: b: c = 0-99798 : 1 : 041387; 

 obliquity of axis (/3) = 94° 57'. 



For the face a we cannot get any more simple symbols than 

 0':&:f e ),fPf. 



Supposing this formula, we find 1 — 



Calculated. Measured. 



m': <r=121 56£ 121 



X : <7 = 152 152 



Comparison with the other crystals of pseudomonoclinic 

 habit leaves no doubt that the face % is identical with the bra- 



chydome ^=§P cc of the normal crystals, and that the differ- 

 ence of the angles x • rn / = 102°, % : m = lQ9°is to be explained 

 by the dislocation of all the faces forming the summit. The 

 angle q : m=104° 20f (calculated on the supposition of the 

 rhombic elements of the Breitenbach enstatite) corresponds to 



