This short but amusing extract entitled ‘Geobabble’ was taken from the newsletter of the Black Country Geological Society number 187, December 2007. It reveals that even minerals can have a sense of humour at this festive time of year.
Petrology deals with the origin, composition, structure, and alteration of rocks. To do this geologists slice up perfectly good wild rocks into extremely thin sections, thin enough for light to pass through the minerals. Once the sections have been glued to glass slides, they can then observe the mineralogical make-up of various rocks. Igneous rocks in particular can be very spectacular under cross polarised filters within the microscope. When viewed under crossed polars some minerals can be very colourful, for example Olivine can be yellow, red and blues; this is called birefringence. To see the different colours from the minerals you must twist them around on the microscope the change in angle shows different colours. Quartz, which is colourless under crossed polars, becomes dark (or black) at certain angles, this is called extinction. Because this extinction (dark area) moves across the quartz crystal in a wave-like fashion as opposed to the whole mineral becoming dark, it is called an undulose extinction.
Those of you who have studied geology will know the feeling when spending hours at a petrological microscope looking at slide after slide as part of a study. Imagine then the reaction of the geologist when a smiley face presented itself. This is exactly what happened to a researcher looking at a thin-section from the volcanic deposits on Santorini. This accretionary lapilli or coarse ash had this happy olivine crystal. About 2mm across, in a groundmass of rectangular plagioclase laths, larger pyroxene crystals and volcanic glass that appears black on the slide. This erupted onto the cinder cone 54,000 years ago.
Written by Bill Groves and Chris Broughton, picture and information from David Miller.