Porphyry (3) Raw Minerals Stones Rocks Collecting

Description

Origin : Italy (Trentino)

Porphyry 101-160 gr - cm 5-11 Raw Minerals Stones Rocks for Collection.

The name Porphyry generically indicates any acidic igneous rock with a porphyritic texture with abundant phenocrysts (over 25% by volume) and a very fine and microcrystalline ground mass. Rocks with these structures can be eruptive veins or hypoabyssal or volcanic, but in the latter case the term is limited to ancient vulcanites, which have undergone a very slow process of devitrification (the so-called paleovolcanites). The color varies from red to brown to purple to gray.
The term porphyry comes from the ancient Greek porphyra, which means "purple". Purple was the color of royalty and for this reason ancient red porphyry was particularly appreciated for monuments and building projects in imperial Rome, so much so that it was called "the stone of emperors".
Being a term of a structural nature, the mineralogical composition can be very varied, therefore it is customary to add the name of a mineral or of the corresponding intrusive or effusive rock as a suffix: for example quartz or non-quartz porphyry, granite porphyry, syenitic porphyry, etc. The term porphyry, widely used in the scientific literature of the past, is now replaced by the name of the intrusive and effusive rock followed by the adjective porphyritic. The porphyries used for statuary, construction and road paving do not always coincide with the petrographic meaning of the term. The so-called ancient red porphyry is actually a porphyritic andesite, while the ancient green porphyry is actually a diabasic andesite.
The largest porphyry extension in Italy is represented by the Permian effusive complex known as the Atesina Porphyry Platform which extends in Trentino-Alto Adige over an area of ​​over 750 km2 with a thickness ranging from 400 to over 1000 m. Other quartz porphyry outcrops are found in the pre-Alps, Elba Island, Monte Amiata, near Civitavecchia (Lazio) and in Sardinia. Quartz porphyries largely transformed by Alpine metamorphism into crystalline schists occupy part of the Ligurian Alps between Cuneo and Savona.
One of the most famous places of extraction and processing is Trentino, especially in the municipalities of the Val di Cembra and on the Altopiano di Piné, where porphyry has a variegated colour, mainly in shades of red, brown, burgundy, gray and purplish.