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Porphyry is a particular type of effusive rock belonging to the so called
"Atesina porphyry base", an enormous complex of volcanic rocks that extends
to the north as
far as the Trentino Alto Adige region.
These rocks are extremely varied and diversified owing to their way of formation. They are
the product of an intense volcanic activity which began 260,000 years ago (in the Permian Period) and
went on for millions of years in a
continuous series of eruptive phases alternated with phases of stasis.
Among the various modes of deposition of these rocks, a particular importance
is to be given to the ignimbritis,
since it was them which gave origin to porphyry.
They are castings of liquid-gaseous mixtures with a thickness varying from 5 to 10 meters and with
a quite constant chemism, until they
constitute the so called ignimbritic unit, that reaches a thickness of hundreds of meters. |
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The currently cultivated porphyry is limited to one of these ignimbritic
units,
classified as riolotic ignimbritis and characterized by very defined vertical cracks and percussion fractures
which are essential for the porphyry processing.
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The cultivable thickness, because of the
lack of slab formation, ranges from 100 to 200 meters. The chemical
composition (over 70% of silica, about 14% of aluminum, 18% of
alkali and small percentages of iron, calcium and magnesium), the
mineralogical composition (quartz, sanidin and plagioclast crystals
and a small quantity of biotite and pyroxene crystals in glassy
paste) and the markedly porphyritic structure determine, besides the
slab formation, the technical characteristics (high compression
break-up load, high chemical- agents resistance, high sliding and
rolling friction) that make porphyry one of the most important
flooring and covering materials in Europe. |
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| The porphyry (riolite) is a magmatic
rock which constitutes one of the last effusive units of the volcanic
complex dating back to the Permian Age (260,000 years ago) in the
Atesina region.
The rock has a porphyritic structure with 35-40% (in volume) of well
crystallized minerals in a glassy or microcrystalline paste. The
minerals are, in decreasing order according to volume and quantity:
quartz, sanidine,
plagiocasio, rare biotite.
The rock is unweathered and it is naturally divided in vertical slabs
with a thickness ranging from 3 to 20 cm.
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Table summing up the physical and
mechanical characteristics of porphyry |
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Chemical Composition of Porphyry
(averaged out of 17 analyses) |
| SiO2 |
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74,14 |
| TiO2 |
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0,16 |
| P2O2 |
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0,08 |
| Al2O3 |
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13,01 |
| Fe2O3 |
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1,34 |
| FeO |
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0,42 |
| MnO |
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0,05 |
| MgO |
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0,36 |
| CaO |
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0,78 |
| Na2O |
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2,70 |
| K2O |
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4,69 |
| H2O |
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0,62 |
| P.C. |
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1.93 |
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compression break-up load |
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Mpa
kg/cm² |
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175
1750 |
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(2) |
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compression break-up load after
gelivity |
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Mpa
kg/cm² |
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211
2110 |
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(2) |
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Inibition coefficient (in weight) |
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°/00 |
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4,0 |
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Bending resistance |
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Mpa
kg/cm² |
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28,2
282 |
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Impact break-up
test |
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kg cm |
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57 |
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Thermal linear expansion coefficient |
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mm/(m
°C) x 10 |
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6.9 |
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Sliding friction wear |
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mm |
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0,84 |
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Normal-elasticity module:
mean elastic module (EM)
Normal-elasticity module:
secant elastic module (ES) |
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Gpa
Gpa
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57,9
58,0
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| Knoop
Microhardness |
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Mpa |
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3240 |
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Notes:
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All tests have been performed according to the following principles:
linear thermal expansion: NORMAL; determination of erasion resistance: RD2234, 1939;
all the other determinations: UNI 9724.
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Determinations performed perpendicularly to the slab plane.
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