How Porphyry is Formed
Porphyry is a type of volcanic rock belonging to the Atesina porphyry platform, an enormous complex of volcanic rocks in Italy's Trentino Alto Adige region.
These rocks, which differ greatly in their chemical composition and formation, are the result of intense volcanic activity which developed with linear eruptions through numerous fractures. This began 260 million years ago in the Lower Permian and carried on for several million more years, alternating between phases of eruptions and inactivity.
Porphyry comes from ignimbrites, which is a deposit in these rocks from the volcanic activity. Ignimbrites are flows of gaseous-liquid mixtures between 5 and 20 metres thick with a rather constant chemical composition and which constituted the ignimbritic unit which is hundreds of metres thick.
At present porphyry is worked only in one of these units (classified as rhyolitic ignimbrite) characterised by very neat vertical fissuring and percussion fracturing which are necessary for working porphyry; because of the lack of stratification, it is possible to work a thickness of about 100-200 metres.
The chemical composition of porphyry (over 70% silica, about 14% alumina, 8% alkali and small percentages of iron, calcium and magnesium), its minerals composition (quartz, sanidine and plagioclase crystals with smaller amounts of biotite and of pyroxenes in a vitreous paste) and its distinctive structure, as well as stratification, determine porphyry's technical characteristics (high compression breaking point, high resistance to chemical agents, high sliding and rolling friction) which make it one of the most important materials for paving and facing.
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