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.
            In photo A the brown biotite crystal, seen towards the top, is 0.5mm long, (N.W. to S.E.)
          The hornblende 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. 
         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.


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