Arthur Holmes — Petrology of North- Western Angola. 269 



maximum dimension being about two millimetres. Intergrowths 

 with, aegirine were not observed, but occasionally a tbin fringe of 

 aegirine, or of minute fibres of limonite, separates the riebeckite from 

 the other minerals. Patchy inclusions of quartz and felspar, minute 

 prisms of zircon, and small grains of ilmenite (?) break up the 

 continuity of the riebeckite individuals, but the mineral has not the 

 spongy appearance which is its customary habit. 



The pleochroism and optical orientation were determined from 

 longitudinal and transverse sections. Faint indications of biaxial 

 interference figures were seen in the latter in very thin sections, from 

 which it was determined that the plane of the optic axis bisects the 

 acute angle between the cleavages. The maximum extinction angle 

 lAc is about 5°. The pleochroism is as follows : X (nearly = c), 

 deep ultramarine blue; Y (nearly = a), yellowish green; Z { — b), 

 smoky greenish blue. Z> X> Y. The colour is generally patchy, 

 and grey and violet tints are sometimes seen in intermediate positions 

 between X and Y. The birefringence is very low. 



In almost every respect, and notably in the position of the optic 

 axial plane, which is parallel to the orthopinacoid, the Angola 

 riebeckite bears a striking resemblance to that of the Quincy granites. 1 

 In the latter case the composition was approximately Na 2 Fe2 (Si 3 ) 4 , 

 42 per cent ; R 4 (Si 3 ) 4 , 56 per cent, in which B, is chiefly Fe 

 accompanied by smaller amounts of Ca, Mn, and Mg ; with excess of 

 2 per cent of Si 2 . This is an unusually high percentage of the 

 li 4 (Si Oa) 4 molecule, but the exceptionally high content of ferrous 

 iron in the Angola rock suggests that there the R 4 (Si 03)4 molecule 

 is equally abundant. Unfortunately the material available was 

 insufficient to allow an analysis of the separated riebeckite to be made. 



^Silgirine makes up about 4 per cent of the rock. The crystals are 

 very irregular and broken in appearance, except where the mineral 

 occurs as tiny inclusions which are elongated along the vertical axis. 

 There is a tendency for small fragmentary segirines to occur in groups 

 which extinguish nearly, but not perfectly, as a whole. Many of the 

 crystals are veined with ferruginous matter, which also collects round 

 the edges in thin fibres and minute acicular projections. The 

 pleochroism is as follows : X (nearly = c), grass green or blue 

 green ; Y (= i), yellow green ; ^(nearly = a), straw to pale yellow 

 green. X> Y>Z. The maximum extinction angle between Xand c 

 is about 9 C , showing that the segirine is not quite pure. 



Zircon is present in extremely minute crystals which are present as 

 inclusions in all the minerals, but more particularly in the soda-bearing 

 minerals. As in the case of rockallite 2 the total amount of zircon 

 present is manifestly insufficient to account for the percentage of 

 zirconia found in the rock by analysis. In this case the zirconia 

 must enter into the composition of the riebeckite. The association of 

 zirconia with riebeckite has been frequently pointed out. 3 Parisite, 



1 Warren & Palache, Proc. Am. Acad. Arts and Sci., vol. xlvii, No. 4, 

 p. 152, 1911. 



2 Washington, Q.J.G.S., vol. lxx, p. 298, 1914. 



3 For references and discussion see Murgoci, Am. Joum. Sci., vol. xx, 

 p. 137, 1905. 



