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 quarry is in staurolite zone Dalradian Kylemore Formation metasediments. This particular specimen is a fine grained staurolite-bearing pelite (a metamorphosed clay-rich sedimentary rock).

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")


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.
The brown "flakey" minerals are biotite.

Photo B is the same section but taken with crossed polarised light. It shows how the matrix consists mainly of biotite and muscovite micas, the latter showing bright interference colours (blues and purples) and quartz (white to grey interference colours). Notice how the staurolite shows mainly grey interference colours since its birefringence, unlike muscovite, is low. 
















 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.
Back to thumbnails.

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.
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