Ferropericlase Inclusion
Ferropericlase Inclusion
A window into the lower mantle, preserved within super-deep diamonds
A ferropericlase inclusion is a minute crystal of ferropericlase — the magnesium–iron oxide mineral with the formula (Mg,Fe)O — trapped within a diamond during its formation in the Earth's lower mantle, at depths exceeding 660 kilometres. Because ferropericlase is a high-pressure phase stable only under the extreme conditions of the lower mantle and transition zone, its presence within a diamond constitutes one of the most reliable diagnostic indicators of ultra-deep, or "super-deep," origin. Such inclusions are of exceptional scientific importance, offering direct geochemical evidence of processes occurring at depths far beyond the reach of any drill or borehole.
Geological Context
The vast majority of gem-quality diamonds crystallise in the lithospheric mantle at depths of roughly 150 to 200 kilometres, within the stability field of peridotite or eclogite. Super-deep diamonds are a distinct and far rarer population, inferred to have formed at depths of 660 kilometres or more — within the transition zone or the lower mantle proper. At these depths, the dominant mineral assemblage differs fundamentally from that of the upper mantle: bridgmanite (formerly called perovskite in this context), calcium silicate perovskite, and ferropericlase together constitute the principal phases of the lower mantle. When a diamond encapsulates one or more of these phases as inclusions during growth, it effectively seals a fragment of deep-Earth mineralogy and carries it intact to the surface via kimberlite eruption.
Ferropericlase has the cubic rocksalt structure (space group Fm3̄m) and is isostructural with periclase (MgO). The iron content in natural lower-mantle ferropericlase typically ranges from roughly 10 to 25 mol% FeO, though the precise composition varies with the local oxygen fugacity and bulk chemistry of the source rock. Under ambient conditions, recovered ferropericlase inclusions are stable and can be analysed by electron microprobe, Raman spectroscopy, and synchrotron X-ray diffraction.
Significance as a Depth Indicator
Among the suite of inclusions used to identify super-deep diamonds, ferropericlase is particularly diagnostic because it has no stable occurrence in the upper mantle under normal conditions. Its recovery from a diamond therefore places the host stone's formation unambiguously within the lower mantle pressure regime — broadly 24 GPa and above. Research published in Gems & Gemology and in peer-reviewed Earth science journals has documented ferropericlase inclusions in diamonds from several kimberlite localities, including stones from Brazil (notably the Juína field in Mato Grosso), South Africa, and Guinea. The Juína diamonds in particular have yielded a remarkable array of lower-mantle inclusion phases, with ferropericlase among the most frequently reported.
Ferropericlase inclusions are often found in association with other lower-mantle phases. Bridgmanite, the most abundant mineral in the lower mantle, occasionally co-occurs, though it typically retransforms to enstatite upon decompression and is therefore identified by its pseudomorphic texture or by residual Raman signature. Calcium silicate perovskite (CaSiO₃) is another co-occurring phase. The assemblage of ferropericlase alongside these phases strengthens the lower-mantle attribution and allows researchers to reconstruct the pressure–temperature conditions of diamond formation.
Host Diamond Characteristics
Super-deep diamonds hosting ferropericlase inclusions are almost invariably Type II stones — that is, they contain negligible nitrogen, or nitrogen in unusual aggregation states inconsistent with the thermal histories typical of lithospheric diamonds. The absence of well-aggregated nitrogen (IaAB centres) in many super-deep diamonds suggests either formation at temperatures too low for efficient aggregation, or residence times and thermal histories that differ markedly from those of conventional lithospheric stones. Some specimens display anomalous infrared absorption features that have been the subject of dedicated spectroscopic study.
Morphologically, super-deep diamonds are frequently rounded, resorbed, or exhibit fibrous internal structure, though these features alone are insufficient for classification. Their identification as super-deep relies on the inclusion mineralogy rather than external form.
Gemmological and Trade Relevance
From a purely commercial standpoint, diamonds containing ferropericlase inclusions are not traded as gem-quality stones in the conventional sense. They are typically small, often cloudy or included to a degree that precludes faceting, and their value lies almost entirely in their scientific content. Major gemmological laboratories, including the GIA, have documented and reported on super-deep diamonds and their inclusion suites, recognising that the scientific significance of such stones warrants careful documentation rather than standard grading. A diamond confirmed to contain ferropericlase or other lower-mantle phases may be acquired by research institutions or retained in museum collections.
For the practising gemmologist, awareness of ferropericlase inclusions is relevant primarily in the context of advanced inclusion studies and the growing field of diamond provenance research. The identification of such inclusions requires techniques beyond standard microscopy — Raman spectroscopy is the principal non-destructive tool — and is generally undertaken in research laboratory settings rather than commercial grading environments.