Garnet-biotite amphibolite from A835 roadside (Grid Ref. NH443 584)
just south of Rogie Falls car park, 2.5km north west of Contin,
Highland. A, B and D were taken in plane polarised light; D
and E under crossed polars.
An amphibolite is a medium grained regionallly metamorphosed igneous
rock composed mainly of hornblende with minor biotite, plagioclase
feldspar and quartz. Garnet can be present, as in this specimen.
photo A the brown biotite crystal, seen towards the top, is
0.5mm long, (N.W. to S.E.)
can be seen to have two prismatic cleavages intersecting at about
56deg. (acute angle) as seen especially in the top right in photos A,
B and C. Hornblendes towards the centre of A, B
and C have been sectioned parallel to one of the cleavages,
hence only one set are seen.
The strong pleochroism
viewed in plane polarised light is evident when comparing A
with B, the colours varying from dark green-brown to pale green-yellow.
C is the same section viewed in cross polarised light. Some
of the quartz crystals show grey to white interference colours.
and E each show two pale pink garnets with quartz
intergrowths. Since their structure is cubic garnets are almost
always isotropic, i.e. they are opaque under crossed polars, as seen