﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



237 



along the a axis. Carlsbad twinning is common, but no twin 

 striations can be detected. The refractive indices are the same as 

 those of the solvsbergite felspars. Small inclusions of the ground- 

 mass are present, in which aegirine and magnetite can be seen. An 

 analysis of the mineral for alkalies, made by Lieut. Angus Mclntyre 

 in 1913, yielded the following results : — soda = 6 - 68 per cent. ; 

 potash = 4 - 51 per cent. 



The phenocrysts are surrounded by a narrow zone of kaolini'za- 

 tion, which in places penetrates the interior along cleavage-cracks. 

 Wherever kaolin is present, calcite is also found, sometimes in 

 granular crystals, sometimes in cryptocrystalline aggregates with 

 kaolin. 



In the ground-mass small laths of anorthoclase are distributed 

 in a mesh, which in texture falls between the trachytic and the 

 intersertal types. Prisms of nepheline of similar dimensions 

 are also present. 



Soda-pyroxenes and amphiboles occupy the wedge-shaped spaces 

 between the felspars, but not completely. The ultimate base of 

 the rock is a mineral, the refractive index of which lies between 

 1"482 and T478, and is identical with that of the natrolite 

 found in the vesicles. Associated with this interstitial natrolite is 

 an obscure isotropic substance. Its refractive index approximates 

 to, but is higher than, that of the natrolite : it contains no 

 chlorine, and is probably analcime or glass. 



With the exception of cossyrite, which forms occasional small 

 phenocrysts, the coloured minerals are all of later crystallization 

 than the anorthoclase and nepheline. Grass-green aegirine is 

 very abundant ; but in most cases the colour is patchy, indicating 

 association with aegirine-augite, which is pale green and gives 

 a high extinction-angle. A few grains of augite, sometimes 

 zoned, are also present. The pyroxenes are frequently included in 

 the monoclinic amphiboles. Cossyrite, however, itself containing 

 inclusions of ilmenite and apatite, is often surrounded by aegirine. 



The pleochroism of katoforite, as exhibited in this rock, is 

 variable ; but the following scheme represents the usual distribution 

 of colour (and also the maximum extinction-angle) : — 



XA« = 33° straw-yellow to colourless. 



T=b smoky red to purple-brown. 



ZAc = 33° greenish -yellow. 



The birefringence is very low, and the refractive index is approxi- 

 mately that of monobrom-naphthalin, l - 66. Some of the grains 

 and wedges of katoforite pass into arf vedsonite, orientated in 

 parallel growth. Its pleochroism is in tints of lavender-grey X ; 

 violet Y ; and yellow-green Z. The extinction-angle 16° differs 

 markedly from that of katoforite, while the birefringence remains 

 about the same. A few individuals that look like katoforite in 

 ordinary light are probably barkevikite, for their polarization- 

 colours are high. The extinction-angle could not be measured, but 

 the refractive index was found to lie between those of monobrom- 

 naphthalin = 1-66 and methylene iodide = 1"74. 



