This article by Guy Lalous ACAM EG summarises a study of the geology, gemmological and physical properties of rubies and pink sapphires from Aappaluttoq, one of the last pristine spots on the planet not far from the arctic circle.
What is the difference between metamorphism and metasomatism?
When rocks change because of an increase in the pressure and/or temperature of their surroundings, it is called metamorphism. When metamorphism is accompanied by a change in the chemistry of a rock, the rock is said to have been metasomatised. Metasomatism involves changes in mineralogy and structure along with the addition and/or removal of elemental constituents.
What are mafic and ultra-mafic rocks?
Both are igneous rocks. In essence, igneous rocks are formed through the cooling and solidification of magma. Ultramafic rocks are igneous rocks with low silica and gas contents which makes them very fluid. Ultramafic rocks are given names depending on whether they are intrusive or extrusive. Peridotite is the name given to intrusive ultramafic rocks, komatiite is the name given to extrusive ultramafic rocks.
Mafic rocks are mostly composed of pyroxene, calcium-rich plagioclase, and minor amounts of olivine. The mafic magmas are somewhat more viscous than the ultramafic magmas, but they are still fairly fluid. Additionally, they contain somewhat more gas than the ultramafic magmas. Gabbro is the name given to intrusive mafic rocks, whereas basalt is the name given to extrusive mafic rocks.
The corundum from Greenland occurs within a phlogopite-bearing metasomatic rock. The rocks at Aappaluttoq have been subjected to high-pressure, high-temperature metamorphism. The mineralisation at Aappaluttoq is hosted by a reaction zone that formed from metasomatic interactions between ultramafic rock (peridotite) and mafic rock (leucogabbro). The peridotite has low silicon oxide (45 wt.%) and contains various chromophore elements (particularly Chromium, but and also vanadium, iron and titanium). The leucagabbro has an aluminium-rich composition.
During regionals metamorphism, fluid interactions between the two differing rock types created a metasomatic reaction zone encompassing part of the peridotite, the leucogabbro and the contact zone between the two units. Upon regional cooling, the reaction zone formed significant volumes of stable phlogopite with corundum. The availability of chromium from the peridotite allowed the substitution of Cr3+ for Al3+ in the corundum, producing its pink to red colour.
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Standard gemmological properties are consistent with metamorphic-metasomatic-type rubies and pink sapphires from other world deposits. Typical inclusion features consist of coarse particles and fine needles of rutile, as well as inclusions of mica, talc, pargasite, cordierite, sillimanite, plagioclase and boehmite. Healed fissures and twinning complete the story. Catapleiite, chlorite, cosalite, dolomite, magnesite, margarite, pyroxene and sapphirine were reported in a previous study.
Spectroscopy in the visible range yielded following data: weak-to-distinct lines were observed at 468 nm and at 475/476 nm along with faint lines at 659 and 668 nm, plus two strong lines at 692 and 694 nm (which appeared as a bright emission line at 693 nm).
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In the mid-infrared region of the spectrum, some rubies and pink sapphires from Greenland showed distinct bands at approximately 3310 and 3075 cm−1 (and weak bands at approximately 2100 and 1980 cm−1). These features indicate the presence of boehmite, which was mostly concentrated along intersection tubules related to twinning and/or parting planes. Such absorption characteristics are helpful not only for identifying the presence of foreign mineral phases, but also for proving that a gem has not been heated.
What about origin determination and treatments?
A genuine untreated ruby is increasingly rare and non-treated gems fetch a substantial premium. Proven origins such as Kashmir for sapphires or Mogok for rubies may considerably contribute to the value of a gemstone. The trade is therefore requesting origin determination from gemmological laboratories. It all started with a microscopic approach in the eighties and developed over time as chemical and spectroscopic criteria were introduced.
Crystal growth characteristics, inclusion identification using a Raman microprobe and analytical tools such as Laser Induced Breakdown Spectroscopy (LIBS) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) have been incorporated into the routine of gemstone analysis.
Consequently, access to chemical elements is gained at very low detection limits. The latest technological development Inductively Coupled Plasma Time-of-Flight Mass Spectrometry (ICP-MS-TOF) allows trace-element characterisation of gemstones for origin determination and treatment detection, is fit for age dating, inclusion studies and high-spatial-resolution chemical mapping of gems.
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The chemical composition of the Greenland rubies and pink sapphires is characterised by relatively high iron contents and comparatively low concentrations of titanium, vanadium and gallium. The iron content separates them from the majority of stones from marble-type deposits.
Rubies and pink sapphires from basalt-related deposits also contain relatively high iron, although correlations between other trace elements can help to separate them from those of Greenland. The iron content of our Greenland stones was similar to that of some rubies and pink sapphires from East Africa. Further work on the trace-element and isotopic composition of the Greenlandic material is ongoing and should prove helpful for origin fingerprinting. Certain mineral inclusions such as cordierite, cosalite and catapleiite may point to a Greenland origin.
Modern mining techniques are planned to maximise production and minimise cost. The mine economics were modelled using only melee-sized rough gem material. Pink sapphire makes up approximately 60-80% of the production, with ruby making up the balance. None of the stones included in the study were heat treated. It should be expected that heated Greenland corundum will become available at some point. The deposit has the potential to make an important contribution to the global supply of ruby and pink sapphire for many years. ■
This is a summary of an article that originally appeared in The Journal of Gemmology entitled ‘Ruby and Pink Sapphire from Aappaluttoq, Greenland’ Christopher P. Smith, Andrew J. Fagan and Bryan Clark 2016/Volume 35/ No. 4 pp. 294-306
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Cover image partially healed fissures composed of groups of isolated negative crystals. Magnification x55. Image courtesy of C. P. Smith