Jeffbenite

Chemical formula: Mg₃Al₂Si₃O₁₂

Jeffbenite is a high-pressure polymorph of pyrope, naturally occurring in the Earth's mantle, characterized by tetragonal crystallization.

## Characteristics Jeffbenite is a mineral from the garnet group, representing a tetragonal polymorph of pyrope. It has been identified as inclusions in diamonds originating from deep layers of the Earth's mantle. It occurs as microscopic, anhedral (not exhibiting its own crystal faces) grains, often co-occurring with other high-pressure minerals. Its identification requires advanced analytical techniques, such as electron diffraction, due to the extremely small size of the crystals. ## Physical Properties As a mineral from the garnet group, jeffbenite is characterized by high hardness, estimated at approximately 7.5 on the Mohs scale, and significant density, around 3.66 g/cm³. It has a vitreous luster. Due to its formation conditions and microscopic size, most of its physical properties are deduced based on structural analysis and comparisons with synthetic counterparts. ## Colors and Varieties Observed natural jeffbenite samples are colorless. No colored or commercial varieties are distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2016 (IMA2016-027). Its name honors two distinguished Earth scientists: Jeffrey W. Harris, a professor at the University of Glasgow, for his contributions to understanding diamond genesis, and Jeffrey B. Benjamin, an electron microprobe specialist from Caltech, who contributed to the analysis of many new minerals. The name is a combination of their first names. ## Applications Jeffbenite has no commercial or industrial applications. Its significance is purely scientific, as it provides valuable information about the conditions in the deep Earth's mantle, diamond formation processes, and the planetary carbon cycle.

Properties

Mohs hardness: 7.5
Luster: Vitreous
Streak: White
Density: 3.66
Cleavage: None
Fracture: Uneven
Transparency: Transparent
Crystal system: Tetragonal

Diagnostic features

## Identification Identification of jeffbenite is impossible without advanced laboratory equipment. Recognition is based on the analysis of its crystal structure using X-ray diffraction or electron backscatter diffraction (EBSD) on a polished surface of a diamond containing the inclusion. It is distinguished from pyrope (its cubic counterpart) by its tetragonal, rather than cubic, crystal structure. ## Differentiation from Similar Minerals The most important feature distinguishing jeffbenite from pyrope is its tetragonal crystallographic system, whereas pyrope crystallizes in the cubic system. Other colorless inclusions in diamonds, such as coesite or some forms of olivine, are distinguished based on chemical composition and crystal structure. ## Crystal Forms Jeffbenite occurs as microscopic, anhedral (xenomorphic) grains or aggregates of grains trapped within diamond crystals. It has not been observed in the form of well-developed, euhedral crystals.

Geological environment

## Genesis Jeffbenite is a high-pressure mineral, forming in the conditions of the Earth's mantle transition zone, at depths of 410 to 660 km. It forms as a result of the phase transformation of pyrope (Mg₃Al₂Si₃O₁₂) under extremely high pressure. Its presence in diamonds indicates that these diamonds formed in very deep regions of the mantle and were then rapidly transported to the surface by volcanic processes, which prevented their re-transformation into pyrope. ## Mineral Associations Jeffbenite co-occurs with other minerals typical of the deep Earth's mantle, which are also found as inclusions in diamonds. These include diamond, ferropericlase, enstatite, and walstromite. Pyrope, from which jeffbenite forms, is also an associated mineral. ## Localities The only confirmed natural occurrence of jeffbenite is the Orapa diamond mine in Botswana. It has been identified as inclusions in diamonds extracted from the kimberlite pipe there.

Rarity

Extremely rare

For collectors

## Quality Criteria Jeffbenite is not a collector's mineral in the traditional sense. Its value is purely scientific. A specimen is a diamond containing a jeffbenite inclusion. For research institutions and museums, the most valuable specimens are those where the inclusion is relatively large, clear, and well-defined, which facilitates its analysis. The presence of other rare high-pressure minerals as co-inclusions further enhances the scientific value of the specimen. ## Market Prices Diamond specimens with confirmed jeffbenite inclusions are not traded on the open collector's market. Their value is difficult to estimate and depends mainly on the interest of scientific institutions. Potentially, they can fetch very high prices at specialized auctions due to their extreme rarity and scientific significance. ## Popular Localities The only known and confirmed locality from which specimens originate is the Orapa mine in Botswana. All studied and described diamonds with jeffbenite inclusions come from there.

Care and storage

## Cleaning Due to its occurrence as microscopic inclusions in diamonds, jeffbenite is not an object that can be cleaned independently. Any cleaning procedures apply only to the host diamond and are performed under laboratory conditions. ## What to Avoid Specimens (diamonds with jeffbenite inclusions) should be protected from extreme changes in pressure and temperature, which could damage the diamond or affect the integrity of the inclusion. All strong chemicals should be avoided. ## Storage Storage pertains to the diamond containing jeffbenite. The safest place is a specialized gemstone storage box or a display case under controlled conditions, protecting against mechanical damage and dust.

Sources

IMA Mineral List (CNMNC)