Bridgmanite Inclusion
Bridgmanite Inclusion
A window into the Earth's lower mantle, preserved within super-deep diamonds
A bridgmanite inclusion is a microscopic crystal of (Mg,Fe)SiO₃ perovskite structure trapped within a diamond that crystallised at depths exceeding 660 kilometres in the Earth's lower mantle. Bridgmanite is widely accepted as the most volumetrically abundant mineral in the planet's interior, yet it is thermodynamically unstable at surface pressures and temperatures, meaning it cannot survive the journey to the surface in any unprotected form. Only the extraordinary mechanical strength and chemical inertness of diamond provides a pressure vessel capable of preserving it. The identification of bridgmanite inclusions is therefore among the most diagnostically powerful observations in modern gemmology and deep-Earth mineralogy alike, providing direct physical evidence that a given diamond formed far below the conventional lithospheric diamond-stability field.
Discovery and Nomenclature
The mineral was formally named bridgmanite in 2014 by Oliver Tschauner and colleagues, honouring Percy Bridgman, the Nobel Prize-winning physicist whose high-pressure experimental work laid the foundation for understanding deep-Earth mineralogy. Prior to formal naming, the phase had been studied for decades under the laboratory designation (Mg,Fe)SiO₃ perovskite. The first confirmed natural occurrence in a diamond was documented from a specimen recovered at Juína, in the state of Mato Grosso, Brazil — a alluvial locality long recognised as a source of anomalous, super-deep diamonds. The confirmation was reported in the literature and subsequently discussed in Gems & Gemology, establishing the inclusion's significance within the gemmological community.
Formation and Geological Context
Conventional gem diamonds crystallise within the lithospheric mantle at depths of roughly 150 to 200 kilometres, under pressures of approximately 4.5 to 6 gigapascals. Bridgmanite, by contrast, is stable only at pressures above roughly 24 gigapascals, corresponding to depths beyond the 660-kilometre seismic discontinuity that marks the boundary between the upper and lower mantle. Diamonds hosting bridgmanite inclusions — commonly termed super-deep diamonds — must therefore have formed in the lower mantle or in subducted oceanic or continental material that descended to comparable depths.
The preservation mechanism is straightforward in principle, if extraordinary in practice: as the host diamond ascended via kimberlite or related magmatic events, the bridgmanite crystal remained sealed within its rigid diamond cavity, shielded from the pressure drop that would otherwise cause it to invert to enstatite or another lower-pressure polymorph. The inclusion thus constitutes a genuine sample of lower-mantle material delivered intact to the laboratory bench.
Gemmological Significance
For gemmologists and gemstone laboratories, bridgmanite inclusions serve as unambiguous provenance markers at the geological level. The GIA laboratories have identified bridgmanite inclusions as diagnostic of super-deep origin, clearly distinguishing affected stones from the far more common lithospheric diamonds. This distinction carries scientific rather than commercial weight: super-deep diamonds do not command a systematic market premium based on their depth of origin, but their identification is critical for researchers studying mantle geochemistry, carbon cycling, and the deep water and volatile budgets of the Earth's interior.
Associated inclusion assemblages in super-deep diamonds frequently include other high-pressure phases such as calcium silicate perovskite, ferropericlase, and occasionally native iron or iron-nickel alloys — a suite entirely unlike the olivine, pyrope garnet, and chromite inclusions typical of lithospheric stones. The co-occurrence of these phases strengthens a super-deep attribution when bridgmanite itself is present.
Identification
Bridgmanite inclusions are identified primarily by Raman spectroscopy, which yields a characteristic vibrational spectrum consistent with the orthorhombic perovskite structure. X-ray diffraction, where sample geometry permits, provides confirmatory crystallographic data. Visual microscopy alone is insufficient for definitive identification, as the inclusions are typically sub-micrometre to a few micrometres in size and may appear as featureless dark or translucent grains under standard gemmological magnification. Synchrotron-based techniques have been employed in research settings to obtain diffraction data from inclusions still sealed within their host diamonds, avoiding the risk of decompression-induced inversion upon exposure.