Cathodoluminescence (CL) microscopy and spectroscopy are luminescence techniques with widespread applications in geosciences. Many rock-forming and accessory minerals show CL characteristics, which can be successfully used in geoscientific research. One of the most spectacular applications is the visualization of growth textures and other internal structures that are not discernable with other analytical techniques. In addition, information from CL imaging and spectroscopy can be used for the reconstruction of processes of mineral formation and alteration to provide information about the real structure of minerals and materials, and to prove the presence and type of lattice incorporation of several trace elements. In the present article, an overview about CL properties of selected minerals is given, and several examples of applications discussed. The presented data illustrate that best results are achieved when luminescence studies are performed under standardized conditions and combined with other analytical techniques with high sensitivity and high spatial resolution.

The 1895 discovery of a petrified tree near Clover Creek in south-central Idaho, USA, attracted worldwide attention and resulted in the naming of a new species of ancient oak, Quercinium pliocaenicum Schuster. For more than a century, the discovery has largely been forgotten, even though specimens reside in reputable museums. Reinvestigation of the locality in 2014/2015 resulted in newly-collected specimens and a wealth of new data. Optical microscopy confirms the cellular anatomy used for the original taxonomic study. X-ray diffraction, scanning electron microscopy, energy-dispersive electron spectroscopy, Raman spectroscopy and cathodoluminescence microscopy reveal details of the mineralization, showing the presence of opal-CT as the primary component, with chalcedony as a lesser constituent. This mineralogy suggests petrifaction occurred in at least two stages, beginning with opalization of cellular tissue, leaving open vessels that became filled with chalcedony during a later mineralization episode. Clover Creek oak represents relict flora growing in a wetter climate before the uplift of the Cascade Range created a rain shadow that caused profound desertification of the inland Pacific Northwest.