They are composed of euhedral to subhedral, coarsely perthitic alkali feldspar
and varying amounts of interstitial quartz (Fig.
The separated but matching adjacent grain boundaries in fractured quartz and residual alkali feldspar
grains in saprolite are indicative of minor pedoplasmation.
A typical rapakivi texture consists of plagioclase-mantled alkali feldspar
megacrysts (ovoids) and two generations of quartz and feldspar.
As a rule, the alkali feldspar
measurements give somewhat higher ages than OSL from quartz, which can be explained by slower bleaching of the feldspar in the sunlight exposure.
39]Ar age determinations on single crystals of alkali feldspar
extracted from pumices found at the top of the blue tuff show that it was deposited 1,468,000[+ or -]16,000 years ago (Stern et al.
The dyke a]so contains abundant phenocrysts and cognate xenocrysts of kaersutite, biotite, and magnetite, and xenocrysts of a]bite and alkali feldspar
Below the cover of glacial till, a series of interbedded sands and shales contain abundant igneous minerals: fresh angular grains of plagioclase, alkali feldspar
, quartz, hornblende, pyroxene, garnet, sphene, magnetite and other opaque minerals and wte, highly kaolinized shale beds contained biotite flakes.
occurs as interstitial grains or antiperthitic inclusions in plagioclase.
This difference is most readily explained by the crystallization of Ca-bearing plagioclase and clinopyroxene in the trachyte magma, and alkali feldspar
crystallization in the rhyolite magma.
Digital images of rock slabs were analyzed to determine proportions of matrix and quartz and alkali feldspar
This episode started with emplacement of the Capo Bianco porphyritic aplite, with alkali feldspar
granite composition, followed by the Nasuto micro-syenogranite.
One of the most complex pegmatites in the region was selected for study in order to investigate the genetic relationship of the dyke to the nearby alkali feldspar
granite, to determine the internal evolution of the pegmatite, and to estimate the physical and chemical properties of the pegmatite-forming fluid.