Thin 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.

This 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.

Courtesy of Dr John Faithful - Hunterian Museum