590 MINERALS AND VEINS OF ^GIRINE-SYENITE, 



• The segirine strongly withstands decomposition, often quite 

 fresh in situations where sanidine and arfvedsonite have entirely 

 succumbed. When zoned, selective decomposition is generally 

 to be noticed, one band only being attached, often to its complete 

 destruction, whilst other parts of the crystal remain perfectly 

 fresh. The various zones have also, to a certain extent, methods 

 of decomposition peculiar to themselves. The outer zone of 

 segirine almost universally passes through a stage of uralitisation, 

 before conversion to what may be more accurately termed decom- 

 position products. 



Uralitisation produces an arfvedsonite-riebeckite type of 

 amphibole, accompanied usually by a small amount of colourless 

 mineral, in basal sections, seen arranged in fine strings along the 

 cleavage directions; this latter substance is largely chalcedon}' 

 (PI. li., fig. 1). In its turn the amphibole suffers decomposition 

 just as does the primary mineral to be later described. 



The volume changes experienced by the conversion of segirine 

 to arfvedsonite would in the case of the theoretically pure 

 minerals be of the following nature : — ■ 



5(Na,O.Fe. 2 03.4Si0. 2 ) = (5Na 2 0.8FeO.Fe 2 3 .16Si0. 2 ) + 4Si0. 2 + 40 

 (Eegirine) (arfvedsonite) 



and assuming specific gravities as determined for the particular 



minerals* described in this paper — 



Vol. of Arfved. _ Molec. wt. of Arfve d. S p. Gr. of Mg. _ 1933-9 2 ; 7 

 Vol. of Mg. Sp. Gr. of Arfved. X Molec. wt. of Mg.~ 3 '35 X 2239"- 5 



and the volume of arfvedsonite works out to be 95-38% of the 

 original segirine. In other words, a loss in volume of 4 - 62 °/. 

 As already noted, a little silica was observed deposited along the 

 cleavage lines; the greater part, however, must be removed during 

 the process, as if all were retained, it is found on calculation 

 that an increase in volume of 10 % above that afforded by the 

 segirine would be necessary to accommodate it. 



In cases where the segirine was observed to pass l-apidly into 

 carbonates, a narrow greenish grey, semi-opaque belt separates 



* Taken in conjunction with theoretical molecular weights, of course only 

 approximate results can ensue. 



