Arthur Holmes — Petrology of North- Western Angola. 325 



interior is chiefly titaniferous augite, with, pleochroic tints of yellow- 

 green, brown-purple, and violet, and a maximum extinction of about 

 40 degrees. Between this and the bordering segirine there is every 

 gradation of segirine-augite. Numerous small crystals of a deep 

 purple colour also occur, but they are never free from a ragged border 

 of segirine. The latter, however, exists alone in long wisps and leaf- 

 like growths. Still smaller crystals and tiny drop-like granules of 

 grey-green augite free from pleochroism are sprinkled over the 

 groundmass. The granules of augite are frequently aggregated 

 together in groups and linear series of varying sizes. 



The amphiboles form a similar series of the barkevikite-arf vedsonite- 

 hastingsite type. The larger crystals are perfectly idiomorphic and 

 sometimes show conspicuous zoning and twinning. The pleochroic 

 scheme is — 



X near to a . . . yellow-green ~| 



Y=b . . . . reddish-brown \Z> Y> X 



Z near to c . . . purple-brown J 



This agrees closely with the pleochroism of barkevikite from 

 Barkevik 1 and Lugar. 2 The extinction angle Z<c is about 10°, 

 and the birefringence is nearly that of ordinary hornblende, offering 

 in this respect a striking contrast to the other members of the 

 amphibole series. Al any of the crystals are bordered with blue-green 

 arfvedsonite, some of which has a red or purple tint in one direction 

 suggesting katoforite. Frequently titaniferous augite and barkevikite 

 have grown together in parallel orientation, with numerous common 

 inclusions of sphene and titanofemte. 



The smaller crystals are lozenge-shaped and possess a deeper 

 colouring and a more intense pleochroism. Some of them are 

 barkevikite, and others, unbordered, belong to arfvedsonite or kato- 

 forite. There are also small" prisms of a green amphibole, which is 

 identical in appearance and birefringence with the hastingsite of the 

 nepheline syenite. The same mineral occurs in association with 

 granular augite, forming an irregular border around the aggregates. 



Sphene in good idiomorphic crystals is abundant, and is generally 

 found clinging to or included in the pyroxenes and amphiboles. It is 

 nearly colourless and therefore the pleochroism is barely perceptible. 



Phenocrysts of nepheline are small though well formed, and are 

 occasionally seen included in or penetrating the minerals already 

 described. A very few clear spaces occupied by analcime may be 

 seen, bordered by the coloured minerals. 



These minerals, with small amounts of ilmenite and apatite, are 

 embedded in a clear transparent groundmass. The absence of felspar 

 in the latter was proved by staining. Radiating needles of nepheline 

 and cancrinite can be readily distinguished, and between them is an 

 isotropic base, which, like the crystals of analcime already mentioned, 

 has a refractive index of about l - 49, and is almost certainly to be 

 referred to that mineral. Further evidence in favour of this diagnosis 

 is provided by the high content of combined water present in the rock. 



1 Brogger, Zeit. Kryst., xvi, p. 412, 1890. 



2 Scott, Min. Mag., xvii, p. 139, 1914. 



