Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 248
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PÁI.I. IMSI.AND
I. AMPHIBOLES
I. Introduction
Amphiboles of two compositional types occur in the Jan Mayen rocks.
These are classified here as basaltic hornblende and kaersutite by compari-
son with Deer et al. (1963).
The basaltic hornblendes are relatively late crystallization phases and
occur in the tristanites and trachytes as relatively big irregular patches
among the smaller groundmass grains or occasionally as an overgrowth on
small clinopyroxene phenocrysts.
The kaersutites occur as occasional xenocrysts in the lavas, as a major
component (ca. 25 vol. per cent) in the coarse-grained hydrous mineral
xenolith, and as scarce microphenocrysts in one trachyte. The kaersutite
xenocrysts have been found in both basic and evolved lavas. In the basalt
lavas the xenocrysts are highly decomposed and only kaersutite remnants
are present in a wide corona of minute opaque oxides and unidentifiable
silicates (Fig. 28). In some cases such corona patches are encountered in the
samples but without containing any kaersutite remnant. The disintegration
of such patches and final assimilation into the basalt liquid is easily
envisaged and even suggested by their similarity to certain textures
observed in some of the poorly crystallized samples. In the trachyte, on the
other hand, the kaersutite is surrounded by a well-defined corona of
relatively coarsely crystallized oxides and silicates (Fig. 44). The kaersutite
of the hydrous mineral xenolith mostly occurs as prismatic crystals. Signs of
alteration and remelting are abundant along grain boundaries of the
kaersutite and both biotite and plagioclase. Analyses of the glasses, repre-
senting this remelting liquid, are given in Table 16. In one of the trachyte
occurrences on the island, Borga (sample: Jan 77 & 79), a few euhedral
microphenocrysts of an amphibole occur as well as the more abundant
biotite phenocrysts. Analyses of this amphibole have not been made but it
resembles the kaersutite closely (Fig. 43) in optical properties.
II. The composition of the amphiboles
As in the apatite and mica analyses only two of the volatile components
were analysed in the amphiboles, i.e. F and Cl. As in the case of these
previous minerals, the water content of the amphiboles has been calculated,
assuming the amphibole to be structurally perfect, containing 24 anions and
the hydroxy-halogen group to amount to 2.000 and to involve only F, C1 and
OH. The possibility of this lattice position being partly occupied by O has
not been considered. None of the iron of the amphiboles has been recalcu-
lated to the trivalent stage and in spite of the statement of Deer et al. (1963)
that basaltic hornblendes are characterized by high Fe203/Fe0 ratios the