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Minerals ~ Porphyry (Rock)
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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
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Quartz-porphyry
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.
Structure
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.
Variants
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. SOURCE
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for Listing
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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
Availability:
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
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