PLEASING PYROPE
Rock&Gem Magazine|December 2020
Italy’s Uniquely Valuable Variety of Garne
MATTEO OBERTO
Brossasco-Isasca pink-violet pyrope, part of the garnet family, has an international scientific value due to its high-depth origin and the particular mineral assemblage included inside the crystals.

The outcrops where this variety of garnet was discovered during the 1980s are in Martiniana Po (Cuneo Province, Northwestern Italy). Throughout the following years, the region has been classified as a “mineralogical geosite.” Martiniana Po exists between Parigi and Case Ramello and is the site of the Pyrope Visitor Center and Museum, which aims to promote and preserve geologic treasure.

The appreciation for Martiniana Po and the pyrope started with a discovery by scientist C. Chopin (1984), who enhanced the coesite (SiO 2) occurrence within the pyrope crystals for the first time. Other researchers focused their studies on the numerous minerals included in these pyropes in the years that followed. The research also led to discovering three new mineralogical species, bearthite, ellenbergerite, and magnesiodumortierite. Soon, Martiniana Po became one of the most essential mineralogical locations in the Piedmont region of northwestern Italy. In consideration of the extraordinary scientific interest, local administrators restricted access to the prominent outcrops. The garnets found in this area can fracture easily, but high-quality gems can be produced from just a few little fragments.

GEOLOGICAL AND GEOGRAPHIC COMPOSITION

Martiniana Po garnets have been found in the Dora-Maira massif, which belongs to the Western Alps’ Penninic domain. Internal Crystalline Massifs (Western Alps) are the Dora-Maira Massif, the Monte Rosa, and Gran Paradiso nappes. These consist of Variscan continental crust intruded by late- to post- Variscan granitoids. Variscan is an event marked by the development of mountains when a collision occurred between Euramerica and Gondwana’s continents to form the supercontinent of Pangaea.

The geological composition upon which Martiniana Po sits consists of a Variscan amphibolite-facies metamorphic basement (mica-schist, impure marble, orthogneiss eclogite), sometimes intruded by late- Variscan granitoids (augengneiss, metagranite, and whiteschist after the Alpine metamorphism). These continental tectonic units show different early Alpine ultra-high pressure coesite-eclogite features (Henry, 1990; Compagnoni et al., 1994) followed by a pervasive greenschist facies re-crystallization (Compagnoni & Rolfo, 2003; Groppo, 2002). Laboratory experiments have shown that the pyrope could be an indicator of high pressure since it would have formed at depths between 50 - 100 km below the Earth’s surface.

DETAILS OF PYROPE

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