Sample:
Medium-grained ‘Central Belt’ quartz monzonite (13H.8a)
Location: Dunmoor Hill NT970177
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 cream coloured plagioclase phenocrysts and black biotite and pyroxene.
Alteration
Altered alkali feldspar showing red-brown colouration, plagioclase altered to alkali feldspar, very littlesericitisation.
Texture
Porphyritic with medium-sized ground mass, ( average 1mm ) with plagioclase phenocrysts ( average 2.8mm ranging to a maximum of 5,7mm ) and some pyroxene phenocrysts (2mm max.).
Zoning in one plagioclase phenocryst.
Mineralogy
N.B. Percentages are of area occupied in this sample measure by pixel count and must be seen as approximate
Mafic content: 9%
Pyroxenes
There is more orthopyroxene than clinopyroxene with pyroxenes as a whole occupying approx. 1.5%of sample. Crystals are much altered to secondary biotite.
Biotite
Occupies approx,7% of sample (av. <1mm, max. 1.5mm) with magnetite in cleavages and at crystal rims. One crystal has many zircon and apatite inclusions.
Opaques
<1% of sample.
Felsic content: 91%
Feldspars
81% of the sample.
Plagioclase
Altered, it makes up 40% of the whole sample and 50% of all feldspar. Occurs mostly in large, often zoned phenocrysts up to 5.6mm in length.
Orthoclase
40% of whole sample and 50% of all feldspar.
Quartz
10% of whole sample occurring in anhedral crystals in the groundmass.
Accessory minerals:
Zircon, apatite