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Phonolites are extrusive equivalents of feldspathoid syenites. In the past, fine-grained rocks of similar composition, but forming intrusions at shallow levels in the crust, were also universally referred to as phonolites. The International Union of Geological Sciences endorsed a new name for these intrusive rocks, feldspathoid microsyenite, but many geologists around the globe continue to (mis)use the term phonolite - indeed, why change something so well-rooted in the professional lingo?
Some phonolites, like the St. Amable sill near Varennes just northeast of Montréal (upper right), may contain cavities (vesicles) and "pipes" lined with minerals common in the igneous matrix of the rock (albite, aegirine, natrolite, etc.), as well as crystals of exceptionally rare minerals known from only a handful of localities worldwide. Such mineralized cavities (amygdales) and "pipes" are products of crystallization from fluids which percolate through, or become trapped in, the phonolite. Other occurrences may comprise plain amygdale-free phonolite, but show spectacular igneous structures - like the phonolite pillars of Devils Tower in northeastern Wyoming (lower right). These pillars are separated by columnar joints that formed when a large body of igneous rock cooled rapidly and contracted.