Garnet-biotite amphibolite from A835 roadside (Grid Ref. NH443 584)
just south of Rogie Falls car park, 2.5km north west of Contin, Highland.
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.
In photo A the brown biotite crystal, seen bottom left, is
0.75mm long. The small white spot at the biotite's top right is a
small zircon crystal. The dark halo surrounding it is due to
radioactive decay of U and Th damaging the biotite structure.
The hornblende can be seen to have two prismatic cleavages
intersecting at 56deg. (acute angle) as seen in the centre and centre
right in photo A. 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 pale yellow to orange interference colours due
to the section being slightly over 30microns thick.
Sections D 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 in E. The bright white patch in A,B and C,
top left, is "holeite", i.e. a hole in the thin section.
