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C |
The sample comes
from Reynishverfi on the south coast near Vik,
Iceland
A: plane polsrised light
B: cross polarised light
C: Close up from where the sample was taken
The rock specimen, collected by one of our members,
is a fresh porphyritic, (i.e. with feldspar phenocrysts), either
dolerite or andesite, depending on the anorthite content of the
feldspars. The predominant minerals are plagioclase feldspar and
pyroxenes, mainly augite, with perhaps minor amounts of olivine, as
well as opaques, probably magnetite or ilmenite.
Under crossed polars, the feldspars show first order white
interference colours with characteristic twinning. The pyroxenes
show higher relief with bright interference colours, e.g. red,
green, yellow and blue. Feldspar and orthopyroxene? phenocrysts can
be seen centre stage.
The exposure, which
appears to be a succession of lava flows is
from where the sample was taken.
More information about this locality will be
welcome: contact ne.geolsoc@ntlworld.com
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A