Structured in the form of a dichotomous key, comparable to those widely used in botany, the mineral key provides an efficient and systematic approach to identifying rock-forming minerals in thin-section. This unique approach covers 150+ of the most commonly encountered rock-forming minerals, plus a few rarer but noteworthy ones. Illustrated in full colour, with 330+ high quality mineral photomicrographs from a worldwide collection of igneous, metamorphic, and sedimentary rocks, it also provides a comprehensive atlas of rock-forming minerals in thin-section. Commencing with a brief introduction to mineral systems, and the properties of minerals in plane-polarised and cross-polarised light, the mineral key also includes line drawings, tables of mineral properties and an interference colour chart, to further aid mineral identification. To minimise the chance of misidentification, and enable less experienced petrologists to use the key with confidence, the key has been arranged to prioritise those properties that are most easily recognised. Designed for simplicity and ease of use, it is primarily aimed at undergraduate and postgraduate students of mineralogy and petrology, but should also provide a valuable source of reference for all practising geologists dealing with rock thin sections and their interpretation.