Sample:
Medium-grained ‘Central Belt’ . (17B.1a)
Location: North of Rig Cairn NT940168
Prepared sample in reflected light
Thin section in plane polarised light (PPL)
Thin section with crossed polarising filters (XP)
Observations
Colour
Red/grey at surface, red-brown in section with distinct cream coloured plagioclase phenocrysts and black biotite.
Alteration
Altered alkali feldspar showing brown colouration, some sericitisation.
Texture
Porphyritic (max. length 5.4 mm, av. 3 mm) with fine-grained groundmass (av. grain size 0.6 mm) with pronounced sector zoning in plagioclase crystals, small amount of micrographic inter-growth between alkali feldspar and quartz.
Mineralogy
N.B. Percentages are of area occupied in the sample measured by pixel count and must be seen as approximate.
Mafic content:
Pyroxenes
15% of sample, poorly formed crystals up to 3.5mm in length.
Biotite
% of sample appearing as ragged flakes, 2.5mm in length with iron ore at rims and in cleavages and with very occasional inclusions of apatite
Opaques
1% of sample.
Felsic content:
Feldspars
Feldspars make up 63% of the sample.
Quartz
18% of whole sample occurring in anhedral crystals in the groundmass measuring up to 1.0mm in length.
Accessory minerals:
Zircon, apatite
Interpretation
The size of the phenocrysts suggests fairly slow cooling probably at depth even though the rock is located quite close to the margin of the pluton.
The high mafic content may be related to the rock’s proximity to the margin.