277 



The pleodiroism for the central and marginal aniphiboles is as 

 follows: 



Centre. Margin. 



c brownish yellow c slightly bluish violet 



b brownish yellow b bine 



a light yellow a nearly colourless. 



It is evident that the crystallographic axes of both am|)hiboles 

 coincide, whilst the plane of optic axes of the central amphibole 

 lies in the plane of symmetry and that of the marginal one is 

 perpendicular to the plane of symmetry. The angle b : c is smaller 

 than that of the typical crossites, whilst only in some of the zonal 

 crystals of crossite, this angle increases to 30°. 



The limit between the different amphiboles in the zonal crystals 

 is rather irregular and there is only a narrow zone of transition 

 between them. 



2. Zonal amphiboles from the Pilandsbergen. 

 In aegirineampliibolefoyaites, which have been collected on Zand- 

 rivierspoort (332) to the South- West of the Pilandsberg complex, 



crystals of zonal am[<hiboles which 

 are tabular on the clinopinacoid, 

 occur. In a section, which was 

 nearly parallel to (010) the central 

 part had a brownish colour, whilst 

 in the marginal zone the colour 

 was more greenish and darker, 

 to darkgreen. The angle between 

 the c-axis and the axis of smallest 

 absorption was 30° for the central 

 and 39° for the marginal zone ; 

 these angles and the colours of the 

 central and the marginal zone are 

 not the same in different, crystals, 

 but the brownish colours are al- 

 ways found in the central and 

 the greenish colours in the mar- 

 ginal zone. 



Sections, in which both central 

 and mai'ginal part are nearly peri)endicnlar to the negative bisectrix 

 show that the angle of optic axes is small for the central part and 

 rather large for the marginal one. The plane of optic axes is perpen- 



Fig 3. Amphibole of Zandrivierspoort 

 (332). Section ± parallel to (010). 



