Geology of gems

Geology of gem deposits is an important topic of research in Nantes, in particular at the lab of Planetology and Geodynamics (LPG).

Geology of emerald

emeraude-chatham-smallEmerald is the green beryl colored either by trivalent chromium (Cr3+) or trivalent vanadium (V3+). Trivalent iron (Fe3+) also gives a green color, but less saturated, so that it cannot give alone a green beryl the "emerald" variety name. In most natural emeralds, Cr3+ and V3+ participate together to the green color, sometimes with Fe3+. Geology of emerald has been extensively developed by a team of geologists at Centre de Recherches Pétrographiques et Géochimiques in Nancy during the 1990's and 2000's. Please consult their works for details.

 

emeralds linked to acid magmatism

emeraude-magmatisme-acide-smallA granitic pegmatite injection within a mafic to ultramafic hostrock (amphibolite, serpentinite...) can make emerald crystallize at the contact between these to chemically contrasted rocks. Pegmatite is rich in silica and brings beryllium, when the host-rock is poor in silica and brings chromium and vanadium. This reaction, called "metasomatism", is possible only when geological fluids (usually water) are present enough to ensure elements transportation. Silicium and beryllium migrate toward the host-rock when chromium and vanadium migrate toward the pegmatite. Emerald crystallizes when beryllium meets chromium and beryllium.

Les émeraudes liées aux failles profondes

emeraude-failles-profondes-smallPegmatites can intrude mafic rocks and no emerald crystallizes, just because fluids are not efficient enough to allow elements migration. On some much later occasion, it can happen that fluids of metamorphic origin circulate again at this contact due to tectonism along deep faults. Metasomatism occurs at this occasion, and emerald crystallizes. This happened for exampleat Piteiras, Minas Gerais, Brazil.

 


Emeralds in black shales

emeraude-colombie-shales-smallThe deposit providing the best quality emeralds is also a geological exception: in the Colombian mines, there are no pegmatites and no mafic rocks, but calcite veins (in white) in fine-grained sedimentary rocks (black), shales. Beryllium, chromium and vanadium all come fro mthe leaching of these black shales.

 

Vanadian emeralds

The formation of vanadian emeralds is one of the subjects we investigated to understand emerald deposits. We demonstrated that sources of vanadium are sedimentary rocks that are rich in either organic matter or iron. This applies to deposits in Colombia, Norway (Byrud), Brazil (Salininha), Pakistan (Gandao), etc. We also examined the properties of Norwegian emeralds to understand their formation. We observed for the first time an assemblage of sulphides in their multiphase inclusions. In addition to providing a criterion for the geographical origin of these emeralds, the sulphides primarily indicate that fluids involved in their mineralization were more reduced compared to fluids in other emerald deposits.
emeraude-IF petitemeraude  Norvege petit
Multiphase fluid inclusions in a Norwegian emerald: They contain cubes of halite and an assemblage of several sulphide, which look like a black grain inside the inclusion. This feature is so far unique to Norwegian emeralds.

Geology of opal

Knowledge of opal conditions of formation has been developed in the last 15 years thanks to several PhDs in our group (Bertha Aguilar-Reyes, Benjamin Rondeau, Eloïse Gaillou, Boris Chauviré) as well as several DUG studies. In particular, these studies have demonstrated that the opal-bearing rock is not necessarily contemporaneous with opal formation. Opal commonly forms by cirucation of fluids which precipitate opal in rocks acting only as containers. It is for example the case in Slovakia (Hungarian opal) where volcanic rocks (andesite) formed well before opal, which is concentrated in veins in the andesine fault system.

opale CharlesX petit opale Slovaquie petit
This opal (left) bought by Louis XVIII of France, then worn by Charles X during his coronation, was found in Slovakia. The geological map (right) illustrates that opal mineralisations (in red) are concentrated along fault systems (dotted lines) in the andesites (volcanic rock; gray).


by contrast, opals from Wollo, Ethiopia,formed shortly after the deposition of volcanic ashes at the Oligocene period (30 million years ago): ashes wheathered during the development of a soil, liberating silica necessary for opal formation. Plant fossils inside opal is a strong indication of opal formation in relation to the develompent of this soil.

opal-Wollo-fossil-smallopal-Wollo-fossil2-small
Plant fossils such as these roots are common in opals from Wollo, Ethiopia.

Geology of opals is one of the most active research topic of the team today.


Geology of corundum

Pr. Bernard Lasnier and Dr. Cédric Simonet clarified the several geological contexts where corundum mineralization can occur. These works were first published in the PhD thesis of Cédric Simonet, then in several publications. In particular, we observed that relative proportions in gallium and magnesium (trace elements) are indicative of the magmatic or metamorphic origin of blue sapphires. For more details, read the article in the journal Lithos.


corindon-Ga-Mg-metam-smallcorindon-Ga-Mg-mag-small
Trace elements help decipher between geological origins of sapphires.

Geology of diamond

A research program in collaboration with the National Museum of Natural History in Paris shown the important role of fluids for gem diamond crystallization. In particular, we described for the first time the nature of a "sugar cube", nearly-cubic cloudy inclusion at the center of a colorless gem diamond. This is a fibrous diamond, very similar to classical fibrous diamond such as the one that covers coated diamonds. In our case, the fibrous diamond is coated by gem diamond: this is the reverse situation of a classical coated. Our observations suggest that gem diamond develops in conditions very close to those of fibrous diamond, only the speed of growth differs between both materials (slow growth for gem diamond, speeder growth for fibrous growth).

diamant petitGem diamond with a sugar cube, nearly cubic cloudy inclusion of fibrous nature, full of carbonate, water, brines and silicate nano inclusions. Sample width: 5 mm.





Hydrogen-rich diamonds (with a cuboid morphology) are indicative of an other medium of growth. They suggest that methan may be the dominant fluid.

Les gemmes du Gondwana

Actu Gondwana

Les éditions du Règne Minéral ont publié en 2012 le second Cahier du Règne Minéral consacré aux gemmes formées à l'époque du Gondwana, qu'on trouve aujourd'hui essentiellement en Afrique de l'Est et en Inde. Nous avons participé à deux chapitres de cet ouvrage coordonné par Gaston Giuliani, géologue de Nancy (CRPG) spécialisé dans la gîtologie des gemmes. Un très bel ouvrage indispensable aux passionnés de géologie, minéralogie et gemmologie, dont la lecture est parsemée d'aventures humaines dans ces lieux d'extraction souvent fort exotiques.

Pour commander : http://www.minerauxetfossiles.com/index.php?page=052.

GONDWANA-COUV1


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