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Water, temperature and pressure: these are the three factors required for the formation of quartzite. Everything began with the water from rivers and seas, capable of eroding and sedimenting sands and gravels with high silicon content. The later diagenesis of these deposits gave birth to sandstones and rudites, thus compacting the sand and gravel granules, held together by a silicic cement. However, the surface of our planet is constantly, slowly

and inexorably changing: orogeny takes place along the convergent boundaries of tectonic plates, causing the formation of mountain ranges. Sandstones and rudites were involved in this orogenic process, undergoing increased pressure and heating to temperatures between 350°C and 550° C; these changes in environmental conditions caused a change in their look, following a process known as low-grade metamorphism. The increase in temperature led to a re-crystallisation of the cemented

granules in the sandstones and rudites, giving birth to a rock composed of a structure of small interwoven quartz crystals: quartzite. We are therefore speaking of a low-grade metamorphic stone, made of 80% quartz and accessory minerals such as tourmaline, feldspar and iron oxide: this variable composition means quartzite can be found in numerous colours in nature, from white to grey, from red to brown and even in blue and green varieties. The high quartz contents make it a resistant and long-lasting stone.

# T HE GENESIS OF QUA RT ZI T E

Quartzite is widespread in the main orogenic chains of the world, from the Western Alps in Italy to Brazil: the genesis of quartzite in the

latter appears to date back as early as a billion years ago. Geologist Elia Migliorini explains how quartzite is born and its main peculiarities.

QUARTZITE IN ART HISTORY

# F OC US ON

Quartzite has been a favoured stone for a long period of human history, from prehistoric times to the modern age: suffice it to say that, for many thousands of years, it was considered an alternative to flint in Palaeolithic communities, and was therefore used to create amygdales and scrapers. The expert hands of ancient Egyptian sculptors used quartzite to build monumental and fascinating sculptures such as the bust of Nefertiti, the Colossi of Memnon and the well-known sarcophagus of Tutankhamun. Looking back to our peninsula, seventeenth century architects in the region of Piedmont chose quartzite to animate the interiors and exteriors of the most beautiful baroque churches in the world: a rare and precious material, this stone proved to be ideal for both structural and decorative applications, thanks to its chromatic variety, which remains its key aesthetic feature.

Elia Migliorini Geologist

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