section of altered basalt, North Glen Sannox., (Hunterian Musuem,
rock number TS17571) Look at this image
to see what this rock looks like in the field.
rock is from the Highland Border Group in North Glen Sannox. These
rocks are often called "spilites" -this is a term used
for basalts which have been extensively altered by seawater, and
enriched in sodium. The texture shown by this rock is at least
in part metamorphic: new minerals include epidote and chlorite
(esp. on the right side of the image): typical of the "greenschist"
facies metamorphism. Other minerals are probably actinolite and
albite - again typical greenschist minerals. The rock does not
show a typical foliated metamorphic texture - these minerals have
not grown while the rock was being deformed. Instead they probably
grew during alteration by hot watery fluids, allowing part of
the original igneous texture to be preserved. Between crossed
polars, the bright birefringence colours of epidote are very obvious.
Note the very low dark grey colours of the chlorite-filled vesicle
on the right.
of Dr John Faithful - Hunterian Museum