148 
Analysis of microcline-microperthite of naujaite-pegmatite. 
I Il Ш IV 
SiO, FERNE 65862 Ma 65'86 65°80 
A Ob PC ET ee 1950 ‘03 18°76 18°75 
еее `55 "55 = = 
MgO .. , . tr. — — — 
Na,0 3°50 21 3°34 3°36 
КО. 11:86 — 12°04 12:09 
Loss on ignition... "38 -- — — 
Total... 100`41 1°56 100 00 100:00 
SD el 258 
I. Microcline-microperthite of naujaite-pegmatite, Nunarsuatsiak. Analysis 
by C. DETLEFSEN. 
Il. Deduction for ægirine-inclusions. 
Ш. Galculated composition of the felspar after deducting 1°56 per cent of 
of ægirine. 
IV. Theoretical composition of a felspar containing 71°61 per cent. KAISi,O, 
and 28°39 per cent. NaAlSi,O,. 
In the felspar-crystals of the naujaite, microcline as a rule 
dominates over albite; very often, however, the central part of 
each crystal is relatively rich in albite while microcline becomes 
gradually more prevalent as we approach the margin. 
As it has been pointed out in the earlier description of 
these felspars! their structure is no doubt due to a primary 
intergrowth of microcline and albite. This is indicated by the 
fact that the composition surfaces of microcline and albite show 
a pronounced tendency to assume a direction parallel to 5 (010) 
which is the predominant crystal face. This feature is common 
to most of the perthites of the Iimausak nepheline-syenites 
(and it has also been noticed elsewhere in rocks of similar 
composition) while it is opposed to the behaviour of the perthitic 
structure in most other rocks in which the perthitic composition 
planes do not follow the direction of any crystal face”. J. H. 
Г. Voer in his interesting study of the crystallization of the 
10 С. Cit- т. 93. 
2106-16. D 180 
