AXEHEAD

The implement was cored and analysed by the South West Implement Petrology Committee in order to identify the rock type.

Report by Roger Taylor SWIPC Petrologist:

1993 CO477 Metabasic 'Greenstone' axe blade, fractured cutting edge

Visual examination

A rough eroded surface with dark raised spots set in a 'feldspathic' matrix of white grains some showing relict crystal form up to 2.5 mm in length and showing separation along former Carlsbad twin planes.

Thin Section

Amphibole -Probably actinolite, pleochroic light bluish green to very pale green as fibrous aggregates and acicular crystals up to 0.75mm long

Pyroxene - Irregular relict grains pale pinkish buff non pleochroic, variably undergoing replacement by amphibole as marginal fringes penetrative 'veins. Some pyroxene remains as small remnant grains within amphibole aggregates.

Mica -Biotite as sparse small medium brown grains and in irregular patches, some associated with ilmenite, 0.1-0.2mmand some appearing to replace amphibole.

Opaque oxide - Irregular platy and skeletal grains, probably ilmenite, up to 1mm.

Feldspar - The section appears to be finished at more than 30µ thickness giving abnormal interference colours and making the altered feldspar difficult to distinguish. It is appears to be heavily sericitised or substantially replaced by amphibole. The rock has affinities with Group I, although the presence of biotite also links it with Group II.

Comment

Examination of the thin section by reflected light with the binocular microscope shows that the relict form of feldspar and the igneous texture is actually quite well preserved. Much of the feldspar exceeds

1 mm and ranges up to 3.5 mm. indicating that the rock is a metagabbro.