This igneous rock is essentially a sodium-rich variety of trachyte or syenite, typically formed through the low-grade metamorphism or hydrothermal alteration of intermediate volcanic rocks. It consists mostly of albite and chlorite or diopside, often displaying a porphyritic texture where larger feldspar crystals sit in a compact matrix. Geologists look for it in ancient volcanic terrains, especially those associated with marine environments.
Heard on the job
The drill core shows a thick sequence of keratophyre interbedded with marine slates.