PITCHSTONE PART I: A HIGH WATER CONTENT GLASSY SOLIDIFIED, VOLCANIC TYPE OF ROCK?

Abstract

The classification of natural glasses as a principal problem is particularly related to pitchstone, obsidian and perlite. On the basis of more then 140 chemical and petrological analyses from both literature and own work common classification criteria are discussed. Typical properties of these glass types are identified and used for an unambiguous classification. It is confirmed that perlite can be understood as a hydrated glass (obisidian). However, under certain conditions, pitchstone was formed directly from a high water content melt. It is futher shown that the perlitic cracks in the microstructure were not caused by fluids. Their origins stem from high thermal stresses in large glassy masses. Water attacked primarily these regions and thus formed the cracking.