![]() It is essential to distinguish between different types of glasses because of their use for dating of geological events, to evaluate the volatile abundances of magmatic source regions, to establish pressure and temperature constraints, and for many other applications. ![]() Natural glass is rare on Earth compared to crystalline rocks due to its specific formation conditions and durability aspects (i.e., glass is metastable and easily altered). Our results suggest that other “unusual glass occurrences” that are claimed, but not convincingly proven, to be of impact origin should be reexamined using the same methodology as that applied here. A suite of Cali glass samples was analyzed using electron microprobe, instrumental neutron activation analysis, thermal ionization mass spectrometry, and Fourier-transform infrared spectroscopy, allowing us to definitively exclude an impact origin and instead classify these glasses as a rhyolitic volcanic glass (obsidian). We used the case of the Cali glass, found in an extended area close to the city of Cali in western Colombia, which was previously suggested to be of impact or volcanic origin, to show that, using a multimethod approach (i.e., combining macroscopic observations, chemical and isotopic data, and H 2O content), it is possible to distinguish between different formation modes. Because glass is often used for the determination of the age of geological events, even if out of geological context, as well as to derive pressure and temperature constraints, or to evaluate the volatile contents of magmas and their source regions, we rely on methods that can unambiguously distinguish between the different types of glasses. This is particularly true for glasses of impact and volcanic origin. Distinguishing between different types of glasses, on Earth and also on the Moon and on other planetary bodies, can be challenging. All these different types of glasses are predominantly composed of silica with variable amounts of impurities, especially the alkalis, and differ in their water content due to their mode of formation. ![]() Natural glass occurs on Earth in different geological contexts, mainly as volcanic glass, fulgurites, and impact glass. ![]()
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