Minerals ~ Porphyry (Rock)
Home ~ Menu
Quartz porphyry from the island of Alnӧ, Sweden. Phenocrysts of clear glassy rounded quartz and
white orthoclase feldspar are set in a fine-grained matrix. Sample is just over 10 cm long

Photo Credit:  Mikenorton

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.


The quartz crystals exist in a fine-grained matrix, usually of micro-crystalline or felsitic structure. In specimens, the quartz appears as small rounded, clear, greyish, vitreous blebs, which are crystals, double hexagonal pyramids, with their edges and corners rounded by resorption or corrosion. Under the microscope they are often seen to contain rounded enclosures of the ground-mass or fluid cavities, which are frequently negative crystals with regular outlines resembling those of perfect quartz crystals. Many of the latter contain liquid carbonic acid and a bubble of gas that may exhibit vibratile motion under high magnifying powers.


In addition to quartz there are usually phenocrysts of feldspar, mostly orthoclase, though a varying amount of plagioclase is often present. The feldspars are usually full and cloudy from the formation of secondary kaolin and muscovite throughout their substance. Their crystals are larger than those of quartz and sometimes attain a length of two inches. Not uncommonly scales of biotite are visible in the specimens, being hexagonal plates, which may be weathered into a mixture of chlorite and epidote.


Click on Image for Listing
Specimens in Mini Boxes

These specimens are mounted in plastic display boxes that measure 2 1/4" x 1 3/4" x 1" deep. Click on the image to see the full listing of available specimens

Price: Marked under Specimens


Each specimen is unique so availability is by image of the piece you select. Orders are taken on a first come basis. We have many more in this size range that are not mounted. Those will be in a separate listing



Webpages  2001-2016
Blue Knight Productions