278 



diciilar to the plane of symmetry in the marginal zone. These am- 

 phiboles resemble those in the shonkinite of the Katzenbuckel 

 (Odenwald) -). 



The relation between these pecnliar amphiboles and the normal 

 types can be seen from the study of the zonal crystals in which these 

 peculiar amphiboles are found together with the normal types. With 

 regard to the common hornblendes, the basaltic ones are character- 

 ized by smaller extinctionangles and smaller angles of optic axes, 

 their alkali- and high ironcontent. 



Similar differences seem to exist between the amphiboles in the 

 zonal crystals with crossite, in which the extinctionangles decrease 

 from 17° to 9° in the brown amphibole. After the turning of the 

 plane of optical axes the margin consists of a crossitic amphibole 

 which is rich in iron and alkalies. In the zonal crystals with crossite 

 and actinolite, the content of sesquioxydes and alkalies strongly 

 increases from the margin to the centre, the turning of the plane 

 of optic axes takes place in a zone of transitional chemical compo- 

 sition. Other amphiboles, which are connected by transitions with 

 the common hornblendes are especially found in igneous rocks rich 

 in alkalies, they are characterized by a larger angle c : c and a 

 smaller axial angle, and are chemically characterized by a high 

 content of iron and alkalies. However their optical and chemical 

 properties are not known in detail. From the description of the 

 zonal crystals of the Pilandsbergen and those of the Katzenbuckel, 

 it is evident, that the turning of the plane of optic axes can result 

 from a small change in the chemical composition of such amphiboles, 

 whilst in other cases (Katoforites) the plane of optic axes remains 

 in the plane of symmetry, whilst the angle c : c can increase from 

 30° to 60° and so passes into that of the arfvedsonites. 



The anoforites from the Katzenbuckel in which the plane of the 

 optic axes is normal to the plane of symmetry (comp. Neues Jahr- 

 buch f. Min. 1910. 1. p. 34), and which in zonal crystals are con- 

 nected with katoforitic amphiboles, differ chemically from the Kato- 

 forites by a lower FeO- and a higher J\lg0 content. The extinction- 

 angles are 20— 27° in the acute angle d. There are amphiboles 

 without the large angle c : c, in which the turning of the plane of 

 the optic axes may be expected in zonal crystals. This is probable 

 for certain amphiboles from pegmatitic segregations in the aegirine- 

 amphibolefoyaites from Buffelspan (585). Their plane of the optic 

 axes was normal to the plane of symmetry, the angle b : c = 14° 



1) W. Freudenberg. Geologie und Petrograpliie des Katzenbuckels im Odenwald. 

 Mitt, der Groszh. Badischen Geol. Landesanstalt. Band V. 1906. 



