A phenocryst is a large crystal in an igneous rock. The name is formed from Greek words phaneros, visible, and kristallos, crystal. Large crystals are designated phenocrysts if they contrast significantly in size with smaller crystals making up the rest of an igneous mineral structure, known as the groundmass.
The term phenocryst is not used unless the larger crystals are directly observable. Thus a phenocryst is a relatively large crystal embedded in the matrix of a finer grained igneous rock. The phenocryst material is a cognate crystal, i.e. sourced from the local mineral chemistry, and is geologically distinct from a xenocryst or xenolith. A xenolith is a piece of alien or xeno rock having a different origin to that of the igneous rock in which it is found. Xenoliths and Phenocrysts are not the same.
This is urban geology. The sill at the base of the buildling on the SE corner of Pirie Street and Gawler Place is formed from a porphyritic granite. The location has provided polished sections, which clearly show the features discussed in the earthcache notes.
Please supply your answers to the following earthscience questions to the cache owner for verification, message or email. You may log your earthcache find immediately.
-Have a look at the phenocrysts. Are they common? What colour are they?
-What average size are the phenocrysts? What shape are they?
-Has the mineral cooled in one stage or more than one? How did you reach your conclusion?