A petrologist uses microscopes and chemical/optical tests to determine the nature of rocks and minerals, in order to aid regional mapping and studies of industrial minerals and mineralisation. Microscope examination of a rock in thin section is the principal technique used to determine the components and features of the rock, the composition of the rock and how it is formed, the geological setting for the rock and the rock name.

A thin section is made for each rock sample, and begins with cutting a rock slice with a diamond tipped saw blade. This rock slice is glued to a glass slide, and ground down with abrasive powder to the thickness of paper so minerals are transparent in the thin section. Another slice of the rock is etched with acid and special chemicals to stain certain minerals for easy identification. The stained slices (or a hand specimens or loose grains) are examined with an ordinary incident light microscope (8 to 40 X magnification) , but thin sections are examined with a transmitted light polarising microscope (60 to 800 X). The observations are typed into a database, and may be supported with photographs and chemical analyses. The samples/thin sections are then catalogued into the Petrology Collection.

Petrological research is also performed on deposits of certain commodities and resource mapping projects (building stone, construction material, copper, gold, platinum, diamond, chromium) and on unusual rock formations. The support work and research work is outcome focussed, striving to acquire reference samples and develop and accumulate knowledge and new ideas, to add value to regional mapping products, and to encourage exploration interest.

The NSW Petrology Collection and Database

The Petrology Collection and the supporting PETROX database is an invaluable resource to the Department and the exploration industry. The collection started in 1884 and has more than 70,000 rocks / thin sections. It is the largest and fastest growing collection in the Department, with more than 1200 samples added per year. The thin section collection is located in Head Office and the rocks of the Petrology Collection are stored at the Core Library, Londonderry. Subject to a few confidentiality restrictions, the thin sections and hand specimens are available for loan.

Please contact michael.bruce@dpi.nsw.gov.au for more information.