Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 74
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PÁLL IMSLAND
that of the biotite phenocrysts ofthe trachytes and tristanites. The biotite of
this rock shows, however, more homogeneity than do phenocrysts from one
trachyte sample to another.
The iron-titanium oxides are euhedral to subhedral ilmenite crystals with
broad and widely separated sandwich type laths of titanomagnetite (Usp62-
59). In some grains the laths are straight (Fig. 57), but in others they have a
wavy form (Fig. 58). This bent, wavy form is hardly a stress indication,
because it only appears in a few grains and no such bending is seen neither
on plagioclase twinning nor biotite cleavage. In addition to these iron-titan-
ium oxides, very scattered small hematite grains occur, associated with
minor yellow and grey sulfide grains.
The apatite forms euhedral crystals included in the iron-titanium oxides
(Fig. 59) or associated with them as described from the rock suite. They
contain both F (0.91 — 1.42 wt. per cent) and C1 (0.47—0.93 wt. per cent).
The scarce clinopyroxene is a salite. Unlike the common titan-salite
phenocrysts of the basic rocks of the rock suite, these are poor in both Ti02
(0.24—0.5 wt. per cent) and AI2O3 (1.42—2.29 wt. per cent) and free ofCr.
The rock is rather vesicular, having both small elliptical vesicles and
bigger irregular cavities. Around some of these vesicles, a small amount of
brown glass occurs (Fig. 60), occasionally partly crystallized. Unlike the
glass of the gabbro descibed above, this glass is homogeneous in composi-
tion.
d. The syenitic xenoliths
Whole rock and mineral analyses: Jan 324
The chemical composition of the syenitic xenolith is closely similar to that
of the trachytes richest in silica. The norm is oversaturated. It contains co as
does one of the trachytes.
Mineralogically these are coarse-grained allotriomorphic granular rocks
(Fig. 61) composed offeldspars with minor amounts of iron-titanium oxides,
apatite, quartz, clinopyroxene and zircon.
The feldspars range in composition from andesine to alkali feldspar. The
most common feldspar is an alkali feldspar ranging in composition from
Or52_42 and occurring mostly as separate grains. Another feldspar is an
andesine core (An39_29) changing towards an anorthoclase (Or23_i2) out-
wards, occasionally surrounded by alkali feldspar of the same composition
as the separate grains.
The quartz occurs as small subhedral to euhedral grains between
feldspars.
The iron-titanium oxides are of two main types: A) Former primary
oxides now highly oxidized and forming anhedral grains of varying internal
textures and compositions. B) Fine-grained recrystallization products of
decomposed mafic silicates.
The internal textures of the former primary oxides are very fine and