Jökull - 01.12.1981, Síða 5

CUMULATIVE
PERCENT
Figure 2. Histogram of approximately 250
fracture vvidths.
Mynd 2. Línurit af um það bil 250 sprungubreidd-
um.
placement observable exceeding 0.1 mm.
The features can be truly linear but also
sinuous. Spacing is very variable. In some
cases the discontinuities bifurcate and rejoin,
creating a dense local network, but it varies
from 2 mm to about 60 mm (analysis of
approximately 250 measurements). Figure 2
shows a histogram of widths and indicates a
preference for 0.02 mm and 0.1 mm. The
smallest width detected in thin section is 0.01
mm.
Examination in thin section provúdes some
further relevant information on the morpho-
logy and fabric. In all the rock types the dis-
continuities are observed to both cross and
follow crystal boundaries. The nature of the
edges varies from very distinct to rather fluffy,
especially in the fine grained rocks. It seems
that the fluffy edges are caused by alteration
where the discontinuities cross glassy patches.
This also causes the sinuosity to be discussed
below. There is no obvious relationship with
vesicles though the discontinuities pass
through them in some cases.
In the Icelandic rocks an open planar frac-
ture is often found immediately beliind a
„weathering rind“ on the prism faces. Break-
ing open the rock usually shows that these
fractures have a red brown coating. The fea-
ture is much too corrlmon to be discounted
although it may be genetically separate in the
time of formation, as discussed later.
Composition of bonding matenal
In thin section the bonding material is
brown to grey or green brown in plane polar-
ised light or red brown towards the prism
edges. Under crossed polarisers it is either iso-
tropic brown or red or birefringent and ex-
tremely finely crystalline. It should be noted
at this stage that small patches of similar
material also occur distributed throughout the
rocks and seem to be an alteration of inter-
stitial glass. Zoning occurs in some discon-
tinuities from the grey brown phase in the
centre to red brown at the rock junction. This
is commonest in the linear examples. Dark
transverse lines occur in some of the broader
examples. Optically this material resembles
phases of chlorophaeite and palagonite desc-
ribed in the literature (Carmichael 1964, Slokes
1971 ,Jakobsson 1978).
Preliminary microprobe analyses were
made on the material from discontinuities in a
tholeiite lava flow from the mountain Esja in
south west Iceland. The rocks of this area are
described in detail by Friðleifsson (1973). The
results for a typical point analysis are given in
Table 1 and appear to be the same for all the
phases. The main chemical features are a high
total Fe content and a low A1 content. Mg
varies by up to 4%. Ca can be very variable
from between 2% and 8%. H..O has not yet
been analysed but could conceivably be
around 20%. The results appear to be very
similar to analyses of chlorophaeite obtained
by Stokes (1971). It isgenerally considered that
the red phase is due to the oxidation of ferrous
iron to ferric iron.
Qualitative X-ray diffraction analysis was
performed on 3 specimens of the same
material. Table 2 shows the peaks obtained for
the untreated samples, on saturating with
ethylene glycol, and on heating to 550°C.
Pretreatment with Mg confirms the behavior.
The results indicate minerals in the vermi-
JÖKULL 31. ÁR 3