A Thin Slice of Rock
Published on 18 August 2023

Looking at rocks under a microscope is one way to examine them. Through explicit and delicate procedures, such as cutting, grinding and polishing, a rock sample can be made into a thin section with a standard thickness of 0.03mm – even thinner than a piece of paper. In addition to looking at the grain size and shape, and the relationship among grains, we can analyse the optical properties of minerals using a polarising microscope.

Each mineral has a specific chemical composition and crystal structure, which exhibit distinct optical properties. The beautiful colours we observe through the eyepiece are interference colours, produced when plane-polarised light passes through the mineral, and light emerges and interacts with the upper polariser of microscope. They are not the same as the colours we see in hand specimens. A combination of optical properties allows us to identify minerals and to interpret their geological story. Integrated with field work, studying thin sections under a polarising microscope adds valuable pieces to the puzzle in geological research. In fact, you can see thin sections of the three main types of rocks in the Earth Science Gallery of the Hong Kong Science Museum. Which thin section is your favourite?