PLEASING PYROPE
Variety of Garnet Helps Put Italy on the Geologic Map
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 (SiO2) 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
Pyrope is a nesosilicate class mineral and is related to the garnet family. Its name derives from the greek Pyropos, defined as “that which looks like fire.” Hence, the color of this crystal is usually flaming red. Pyropes in nature appear in metamorphic rocks, which form lenses sandwiched between mica schists and minute gneiss. The Martiniana Po pyrope crystals illustrate this well, as they are often found in the shape of rhombododecahedral crystals, with dimensions ranging from 0.2 mm to over 25 cm in diameter. The external surface is opaque and encrusted with white mica and other minerals. The color of pyrope in this region varies from whitish to light pink, and occasionally brown to blue, but mainly present as pink. The crystals have an almost rounded appearance with smooth edges.
This pyrope’s chemical formula is primarily Mg3Al2 (SiO ) , consisting of three atoms of magnesium, two
of aluminum, three of silicon, and twelve of oxygen. The chemical composition of the Martiniana Po pyrope was asserted by Chopin, who established that these crystals are made up of a solid mixture of pyrope and almandine, with the majority (98%) pyrope. The internal fragments of these crystals show variable colors related to the stage of transformation. Worthy of interest is the gem variety of the garnet described, offering a different color due to the main crystal’s fragment position. Even though these garnets tend to be variously fractured and difficult to cut into gems, some transparent fragments have already been found with a color ranging from pink to blue. These factors and more make collecting the Brossasco-Isasca garnets even more fascinating. In addition to the mineralogical value, there can be a value in the field of precious stones.
Coesite-bearing pyropes are among the species with the most significant scientific interest. Coesite consists of the crystalline monoclinic modification of silicon dioxide, which formed at pressures above 25 kbar and a depth close to 100 km. It can still be found with microscopic dimensions into the smaller pyrope crystals or in the peripheral portions of larger crystals in the form of nodules. The “radial cracking” associated with coesite is a phenomenon, which affects the surrounding garnet. This consists of a system of fractures with a radial pattern.
Among the other species included in the pyrope, the ellenbergerite is also of great interest, as Martiniana Po is the original type locality. It’s dark purple magnesium, aluminum, and titanium silicate. It can be seen in prismatic, sometimes isolated, or grouped crystals with a hexagonal section. The presence of titanium and zirconium may change the violet color of this mineral. Other notable minerals are bearthite (Chopin et al., 1993) and magnesiodumortierite (Chopin et al., 1995), magnesium, aluminum, and titanium silicate, and often presents as pink prismatic crystals, which can reach a few millimeters in length. Additional minerals known to appear in conjunction with pyropes include zircon, phengite, monazite, kyanite, talc, paragonite, and vermiculite.
Thanks to the zircon minerals, Martiniana Po’s pyrope can be dated. In fact, Tilton et al. (1989-1991) made the first studies, which indicated an oligocene age (38-35 and 35-31 million years ago) based on geochronological methods (U-Pb on zircons, Sm-Nd and U-Pb on pyrope, ellenbergerite, monazite, and phengite). Further details were carried out with studies on single zircon crystals (Gebauer et al., 1997), which provided a formation age of 35.4 million years ago. Another confirmation came from Rubatto & Hermann’s (2001) studies, using a method measuring individual zones of titanite growth in the marble silicate nodules, indicating a peak age of 35.1 million years ago.
As illustrated in this article, when visiting my home country of Italy, my advice is to survey along the slopes of the Po (Martiniana Po; there is a natural park) and Varaita valleys (Vallone di Gilba; collecting is free). Also, be sure to spend time in the local rivers, where you may find some boulders, in which the scientifically essential and culturally valuable pyrope garnets discussed here can be found.
Matteo Oberto is a young Italian gold prospector. He has a degree in gold ore exploration and his primary passions involve precious metal surveying, the study of geology, and discussing and writing about the topic of prospecting.
REFERENCES
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