There are about 70 000 rock thin sections and over 2000 polished block samples stored along with their hand specimens at the W B Clarke Geoscience Centre, Londonderry.

Petrographic collection

Microscope examination of a rock using a thin section is the principal technique used to determine the composition of the rock, how it is formed, and its geological setting.

A thin section of a rock is made by gluing a small slice of the rock to a glass slide, then grinding the rock slice until it is 0.03 mm thick. At this thickness, the minerals in the rock range from being translucent to opaque and exhibit a variety of distinguishing characteristics as they interact with plane- and cross-polarised light. The mineral assemblage and textures in the rock, as seen under the microscope, enable its identification and helps interpret the processes that formed it.

Regular thin sections, made using a cover-slip, are for viewing under light that is transmitted through the sections. Polished thin sections do not have a coverslip, and are made for viewing under reflected light or transmitted light. Reflected light is best utilised to look at opaque minerals such as oxides or sulphides.

Descriptions and photos of the department's thin sections will be available through MinView. These are closely associated with Field Observations also accessible through MinView.

Polished block collection

Reflected light microscopy is used to identify opaque minerals such as ore minerals in small, polished rock samples. Small, cm-sized rock samples are bonded in epoxy and polished as small round blocks. This technique allows the characterisation of ore minerals without needing to make a polished thin section. Such petrographic analysis is useful to help determine the order in which individual minerals were deposited, for example, during investigations into individual mineral deposits.

Access to collections

Subject to confidentiality restrictions, thin sections, polished blocks, and their corresponding hand specimens are available for loan. For further information please contact