Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 27

4. ALTERATION AND OXIDATION
A. GENERAL ALTERATION
The analysed samples are relatively fresh and show only small signs of
alteration.
Only two of these samples contain visible calcite in thin-sections, one
basalt (Jan 29) and one trachyte (Jan 171). These samples are the only ones
analysed for COz and contain 0.20 and 0.78 wt. per cent respectively. In the
field secondary minerals in vesicles are very scarce. Only in one locality can
they be said to be abundant, but even here the vesicles are only partly filled
by (unidentified) zeolites and calcite.
On the shore of the island one may find small snow-white pebbles
weathered out of rocks and polished in the surf. In one case these were found
inside a basalt boulder, but they were never found in rocks in situ.
XRF-diffraction analyses show that these are almost pure magnesite
(MgCOs). The 100-intensity peak of calcite is just visible on the diífracto-
gram indicating a very low calcite component in the mineral.
Some of the fme-grained xenoliths of volcanic rocks occurring on the
island are considerably altered. The alteration is shown by the occurrence of
smectite in the groundmass and calcite in cavities and cracks.
Many of the analysed samples show secondary oxidation. This is espe-
cially found in the groundmass minerals. Reflection microscopy reveals
trellis oxidation exsolution ofgroundmass minerals and microphenocrysts of
the iron-titanium oxides of several basic rock samples. In some of the
intermediate rocks iron-titanium oxides are altered to metailmenites.
In some of the ankaramites secondary oxidation has lead to separation of
oxides on a fine scale in both groundmass and phenocryst olivines.
Biotite phenocrysts occuring in some of the trachytes are altered to a fine-
grained aggregate of oxides and silicates.
In the whole rock analyses the Fe203/Fe0 ratio ranges from 0.07 to 2.29
for the basic rocks but from 0.38 to 14.0 for the intermediate ones.
B. “CORRECTION” OF OXIDATION
To prevent an undesirable effect on the norm of the rock analyses,
numerous authors choose to “correct” the iron ratio (Fe203/Fe0) of the
analyses. In most cases it is attempted to approach the iron ratio conditions,