Volcanic Tuff
This sample of phonolitic welded ignimbrite comes from the Arico ignimbrite, Tenerife.
In thin section this pyroclastic rock contains fragments of alkali feldspar and biotite with minor magnetite, hornblende and diopside, flattened pumice clasts (fiamme) and lithic fragments in a matrix of recrystallized volcanic glass shards.
Click http://www.virtualmicroscope.org/content/ignimbrite-p17017 link to open resource.