A |
The photographs here are taken from a thin section
of a staurolite schist from Claggan quarry, County
Galway, Republic of Ireland. (Irish
Grid Ref: M 011506). The staurolite bearing pelites in this area are part of a regional metamorphic sequence found in high grade schists and gneisses of the Dalradian Supergroup in the Connemara district of County Galway, western Ireland. Barrow (1853-1932) proposed a sequence of zones from low to high metamorphic grade, each grade with an "index mineral". Barrow's proposed sequence from low to high metamorphic grade in rocks of pelitic composition in the Dalradian Supergroup are as follows: chlorite, biotite, garnet, staurolite, kyanite, sillimanite. (cf. Roger Mason's "Petrology of the Metamorphic Rocks") |
B |
Photo A,
taken with plane polarised light, shows high relief porphyroblasts
of staurolite showing typical pale yellow colours. The lozenge shaped
crystal in the centre is 0.6mm in length (left to right). Shades of
yellow vary since staurolite is pleochroic.
|
C |
|
Photo C is a
highly magnified part of the above showing the lozenged shaped porphyroblast
as seen in the centre of photo's A and B. Staurolite's typical yellow
colour is apparent. The diameter of this photo is just over 1mm. |
D is the same photo as C except taken with crossed
polarised light. Again, the low birefringence of staurolite can be
seen as well as the other minerals mentioned above. |