Quartz-porphyry

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz.

The ground-mass is finely crystalline and to the unaided eye has usually a dull aspect resembling common earthenware; it is grey, green, reddish or white.

A large number of the finer quartz-porphyries are also in some degree silicified of impregnated by quartz, chalcedony and opal, derived from the silica set free by decomposition (kaolinization) of the original feldspar.

This re-deposited silica forms veins and patches of indefinite shape or may bodily replace a considerable area of the rock by metasomatic substitution.

The opal is amorphous, the chalcedony finely crystalline and often arranged in spherulitic growths that yield an excellent black cross in polarized light.

[3] In other cases these two minerals are in graphic intergrowth, often forming radiate growths of spherulites consisting of fibers of extreme tenuity; this type is known as granophyric.

Fluorite and kaolin appear also in these rocks, and the whole of these minerals are due to pneumatolytic action by vapors permeating the porphyry after it had consolidated but probably before it had entirely cooled.

Many of the older quartz-porphyries that occur in Paleozoic and Pre-Cambrian rocks have been affected by earth movements, and have experienced crushing and shearing.

In this way they become schistose, and from their feldspar minute plates of sericitic white mica are developed, giving the rock in some cases very much of the appearance of mica-schists.