Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 147

MINERAL CHEMISTRY AND RELATIONSHIPS
143
II. Spinels
The minerals of the spinel solid solution series mostly follow the wehrlite
minerals and thus occur in greatest amount in the ankaramites and the
wehrlite. In the ankaramites they occur mostly as rounded to euhedral
inclusions in the wehrlite minerals or as grains attached to the wehrlite
minerals and occasionally as discrete phenocrysts. In the wehrlite xenoliths
they occur as more or less rounded discrete grains and inclusions. The
relatively scarce spinels of the basalts appear as discrete rounded pheno-
crysts and small euhedral inclusions in titan-salite and unstressed olivine
phenocrysts. They differ in composition from those of the ankaramites as
will be shown in the next section. The spinels represent early crystallizing
phases as shown by their mode of occurrence and the fact that no spinel
inclusions have been found in the plagioclase, which is a reladvely late
phase.
A late crystallizing spinel occurs in these rocks as well, but in extremely
small amounts. This is Fe and Ti enriched relative to the early spinels and
occurs as a thin zone surrounding the other spinel grains when these are
enclosed by groundmass or as small discrete groundmass grains. These late
spinels may be further mantled by an iron-titanium oxide frequently
showing ilmenite lamellae of trellis oxidation exsolution. The spinels them-
selves show no signs of exsolution or alteration. This zoning is not found
where the spinel grains are enclosed by the wehrlite minerals or attached to
these. In these cases the crystals are fairly homogeneous from core to edge.
There is a compositional gap between the íarly spinels and the
titanomagnetites and the scarce, late spinels tend to bridge this gap as
shown by Fig. 88.
a. The composition of the spinels
The major cation compositions of the spinels are shown by their position
within the spinel prism (see Irvine, 1965) in Fig. 89. Representative analyses
are given in Table 21. The compositions are within the frame marked by
published spinel analyses, but the evolutionary trends are partly in disagree-
ment with other trends as will be shown.
Eleven elements were analysed in the spinels. Except for the major
elements of the spinel structure, Mg, Fe, Al, Cr and Ti, six minor elements,
Si, V, Mn, Ni, Ca and Zn have been determined in most of the samples (Zn
has, however, been determined in few samples only). The total Fe of the
analyses has been distributed between Fe3+ and Fe2+ by calculations, based
on oxygen balance, assuming perfect stoichiometry in the spinel structure
(see Hamm & Vieten, 1971; Finger, 1972; and Neumann, 1976). That this
assumption is valid is strongly indicated by the good correlation of FeO and
Fe^Os (calculated wt. per cent) and other measured and calculated para-
meters (see e.g. Fig. 90 and Fig. 91). This correlation is much better than