roghadal

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This specimen (magn x50) was collected by one of our members, Mavis Gill, near Roghadal (NG050 830), Outer Hebrides. (The web editor is entirely responsible for its interpretation). It is a Pelitic Garnet-Hornblende-Cordierite Gneiss. Gneiss is a general term referring to metamorphic coarse grained banded rocks. Banding is due to separation of dark and light minerals. Dark minerals usually biotite, hornblende & pyroxenes. Light minerals, quartz & feldspars.
Photo A, plane polarised light, shows pale pink garnet, e.g. just right of centre, and pleochroic green and brown hornblende. The colourless areas consist of quartz and cordierite. The original rock would have been sedimentary consisting of quartz and clay minerals.

Photo B shows the same section in cross polarised light. Just north west of centre is a mineral with lamellar twinning; what appears to be a feldspar. It is, in fact cordierite. Just right of centre is a garnet. Being cubic in habit, it is isotropic, i.e. it is opaque under x polars. C and D are photo's taken under a scanner where the gneissose banding is clearly displayed and accentuated by the paler coloured minerals.

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This specimen (magn x50) was collected by one of our members, Mavis Gill, near Roghadal (NG050 830), Outer Hebrides. (The web editor is entirely responsible for its interpretation). It is a Pelitic Garnet-Hornblende-Cordierite Gneiss. Gneiss is a general term referring to metamorphic coarse grained banded rocks. Banding is due to separation of dark and light minerals. Dark minerals usually biotite, hornblende & pyroxenes. Light minerals, quartz & feldspars. Photo A, plane polarised light, shows pale pink garnet, e.g. just right of centre, and pleochroic green and brown hornblende. The colourless areas consist of quartz and cordierite. The original rock would have been sedimentary consisting of quartz and clay minerals.	Photo B shows the same section in cross polarised light. Just north west of centre is a mineral with lamellar twinning; what appears to be a feldspar. It is, in fact cordierite. Just right of centre is a garnet. Being cubic in habit, it is isotropic, i.e. it is opaque under x polars. C and D are photo's taken under a scanner where the gneissose banding is clearly displayed and accentuated by the paler coloured minerals.