Porphyry can only be quarried in a limited number of places. It’s a stone that has been known and used since ancient times. Important relics and monuments in porphyry have been discovered at Assyrian-Babylonian, Egyptian and Roman sites, the cradles of civilization. Since the time of the Romans, porphyry has become one of the most important varieties of stone in Europe. The rock formed under such intense heat and pressure (making it more dense than granite) that was ideal for paving Roman roads, including those that traversed the Alps and became the pavements of Paris. Because of its density, porphyry absorbs almost no water at all. That makes porphyry resistant to cracking — water can’t get in to freeze and expand. That trait is useful where changing seasons create multiple freeze-thaw cycles every year. Because it doesn’t absorb water, the stone dries quickly, which makes it ideal for use in wet areas such as showers and pool decks. It is naturally slip-resistant — the material’s natural texture meets requirements set by the Americans With Disabilities Act. Moreover, because it’s incredibly hard, it resists scratches in addition to being resistant to chemicals. It is further distinguished by the richness of its coloration — porphyry means purple in Greek — and its characteristic purplish-red or greenish-brown swirled patterns have made it a popular choice over centuries as a paving material.
The rock is composed of a micro crystalline ground mass with phenocrysts of average size 0.5 – 1 mm (maximum 5 mm) contained in it. In the phenocrysts the following principal constituents are found (in decreasing order)
- Feldspar, in rather muddy crystals with inclusions of opaque minerals and sheet silicates;
- Quartz, in normally clear crystals, larger than average;
- Chloritized biotite, with abundant opaque inclusions.
The rock could even be called a rhyolite or, based on slightly old-fashioned terminology still in use, a quartzite porphyry.
By the term porphyry we mean the extrusive ignimbrite which is dated in the Lower Permian. From a graphic, petrological point of view porphyry is the extrusive equivalent of granite and thus has a variable composition. It passes from basic composition like porphyrites and trachytes, to the more acidic compositions of rhyodactites and rhyolites. Porphyry is an extrusive rock originating in the Lower Permian, belonging to the Porphyric Atesina Platform. The rock is extremely varied as a direct consequence of extensive tectonic activity which resulted in linear eruptions along numerous fractures. The porphyry that is most often used for paving is stratified – having formed in layers that allow for easier extraction. This is also what gives porphyry paving its variable thickness.
The geological situation was affected by prior erosion processes, demonstrated by the presence of basal conglomerates, which gave rise to decompression and facilitated the rise of the magma. The volcanic activity continued for millions of years with successive eruptive and stable phases. The eruptive structures were extremely varied including necks, dykes, sills, ignimbrites etc. On the porphyry platform it is possible to find the results of reworking which indicate pauses in the phases of magmatic activity, and the presence of seams of predominantly rhyolitic material containing pink feldspar phenocrysts and larger crystal structures. These features document the rise of the magma after a stable phase. The eruptive activity lasted for tens of millions of years and was accompanied by a series of downward faulting and tectonic activity thus producing an extremely complex geological area. The area has a very varied morphology due to selective erosion, in particular associated with the more errodable rocks like tuffs and conglomerates, while the ignimbrites result in vertical cliffs or, after glacial erosion, “mouton” mounds.
Among the extrusive rocks present, the “ignimbrites” are particularly important because from them we obtain the porphyry suitable for cutting and paving. The ignimbrites originate from the collation of extremely fluid liquid-gas mixtures at high temperature. Being rich in gas and having a more acid composition they are more viscous and so extend over a wide area. From this we can explain why the ignimbrites have a sub-vertical, slab-like structure with parallel strata. This slab effect contributes to the workability of porphyry. The thickness of a typical unit is between 100 – 200 meters, with an area of about 7,500 km2, of which only about a quarter outcrops while the rest has been covered by later deposits. The “stratification” of these rocks makes it possible to split them into slabs along the parallel planes which are sub-vertical or steeply inclined.
The chemical composition of the Mexican porphyry is as follows:
- Silicon oxide (SiO2): 73.68%
- Aluminium oxide (Al2O3): 12.05%
- Titanium dioxide (TiO2): 0.05%
- Iron oxide (Fe2O3): 2.62%
- Calcium oxide (CaO): 0.63%
- Magnesium oxide (MgO): 0.72%
- Potassium oxide (K2O): 5.77%
- Sodium oxide (Na2O): 2.06%
- Loss on heating (PF): 1.95%
The mineralogical composition comprises principally quartz, sanidine and plagioclase, and as accessories biotite and vitreous material. The porphyric structure encourages stratification and improves the technical characteristics (high breaking load under pressure, high resistance to chemicals, etc..). These characteristics make porphyry one of the most important materials for paving and cladding. As previously stated, throughout the Lower Permian volcanic activity occurred between periods of stability-this makes it possible to identify the difference between workable masses and unworkable masses from the variation in colour, composition, inclination and grain characteristics. In particular the colour is an important indicator for the identification of rock types which are suitable for being worked. In fact the colour varies from a pinky grey to a darker shade due to the absence of quartz. The first type are suitable for industrial use, the second rather less so, if they are not completely useless. This variation in colour is due to the variation in composition. The more basic composition is darker while the more acid is lighter in shade and more suitable