| Mitchell Isaacs from Australia wrote:
be surprised if there's a bit of calcite in the dyke... The dark spots
are probably xenoblasts - although, could be weathered phenocrysts.
xenoblasts are basically bits
of rock that have broken off and mixed into the magma as it has
moved. xeno=foreign, they are like foreign bodies. (not crystallized
from the surrounding magma). Phenocrysts are large crystals...
...the orangey sort of colour
is largely from K-spar (orthoclase) weathering.
...the sample described as "mafic"
[Ed. in sample photos below] is actually about as far from mafic
as you get. In the close up, I can make out Qtz, K-spar, I would
guess there would be some plag, and there are a few black specks.
A mafic sample would be dark in colour - in a mafic enclave in
a rock like this, you would expect mostly biotite & hornblende
(which is what I suspect your dark spots are largely composed of).
Another poster suggested tourmaline
- however, hornblende as your non-biotite dark mineral is much
more likely. Tourmaline is often associated with pegmatitites,
which are much coarser grained that this dyke.
...it looks like you have a
felsic dyke, broadly granitic in composition, with numerous mafic
KMR from Austria (I think) says:
describe sounds very much like a felsic dike. Often, the very lightest elements
in the Magma accumulate at the top of the Magma-Chamber, thus intruding
first into the above crust , as felsic dikes. Those typically show coarse
crystallisation. Granitic composition is also typical, as you described (feldspar,
quarz, mica), ... I'd speculate that the black minerals which are no[t]
biotites, are black turmaline or some amphibolites.