THE PHYSICAL CHEMISTRY OF THE CRYSTALLIZATION 
AND MAGMATIC DIFFERENTIATION 
OF IGNEOUS ROCKS 
J. H. L. VOGT 
Trondhjem, Norway 
III 
ON THE QUARTZ, CRYSTALLIZING AT A LATE STAGE, IN QUARTZ- 
BEARING NORITES, GABBROS, SYENITES, DIORITES, ETC. 
In the rocks here mentioned, where the quantity of quartz does 
not surpass 5 per cent or occasionally—especially in some quartz- 
diorites*—somewhat more, the quartz, as is well known, appears as 
Zwischenklemmungsmasse, or mesostasis, indicating a very late 
stage of crystallization. If we for convenience’ sake only concern 
ourselves with the gabbroidic rocks, we find this depending on the 
fact that in a complicated system (Ab+An:ferromagnesian meta- 
silicates [with other pyroxene components]:Qu), a great deal of 
plagioclase and pyroxene will crystallize at an early stage if the 
Qu component is present only in small quantity. By this means 
the quantity of the Qu component in the mother-liquid increases, 
and the quartz can only commence forming when a complicated 
eutectic boundary between Qu and Ab+An and ferromagnesian 
silicates has been reached. 
In this manner the quartz will fill the intervening spaces between 
the already formed plagioclase and pyroxene individuals. At 
this late stage of crystallization, however, we have not, as occa- 
sionally assumed by some earlier investigators, a crystallization of 
quartz alone, but, on the contrary, of quartz simultaneously with 
some plagioclase and ferromagnesian silicate. ‘The fact is that the 
quartz in the mesostasis often forms a pegmatitic or granophyric 
intergrowth as well with the plagioclase as with the pyroxene in 
t Rocks with acid plagioclase, with at least 15 per cent quartz, I do not include 
in the group of diorites. 
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