Xiexiandeite

Chemical formula: SiO<sub>2</sub>

Xiexiandeite is an extremely rare mineral of the spinel group, an iron and chromium oxide, found as microscopic inclusions in diamonds.

## Characteristics Xiexiandeite is a very rare iron and chromium oxide belonging to the spinel group. It is the natural analogue of synthetic iron(II) chromate. It occurs as microscopic, black, opaque grains with a metallic luster. It typically forms small, well-formed (euhedral) or partially formed (subhedral) crystals with a habit typical of spinels, i.e., octahedral. ## Physical Properties The mineral is characterized by a metallic luster and is completely opaque. Its density is calculated to be 4.96 g/cm³. The hardness has not been precisely determined, but like other spinels, it is a relatively hard mineral. ## Colors and Varieties Xiexiandeite is uniformly black. No color varieties or commercial varieties are distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2007. Its name honors the Chinese mineralogist and petrologist Xie Xiande (born 1934) from the Chinese Academy of Geological Sciences, in recognition of his contributions to high-pressure mineral research. The type locality where this mineral was first identified is the Luobusha ophiolite in Tibet. ## Applications Due to its extreme rarity and microscopic size, xiexiandeite has no commercial or industrial applications. It is solely an object of scientific interest, providing valuable information about processes occurring deep within the Earth's mantle.

Properties

Luster
Metallic
Density
4.96
Transparency
Opaque
Crystal system
Isometric

Diagnostic features

## Identification Identification of xiexiandeite is impossible without advanced analytical techniques. Due to the microscopic size of the grains, it requires the use of an electron microprobe with an energy-dispersive X-ray spectrometer (EMPA) to determine the chemical composition and X-ray diffraction (XRD) to confirm the crystal structure. A key indicator is its specific occurrence environment – as an inclusion in diamond originating from Earth's mantle rocks. ## Distinguishing from similar minerals Under a microscope, it can be confused with other opaque minerals of the spinel group, such as chromite or magnetite, which may occur under similar conditions. Definitive differentiation is only possible based on precise chemical composition analysis. ## Crystal forms It forms microscopic, isometric crystals, most often in the form of octahedra, which is typical for minerals of the spinel group.

Geological environment

## Genesis Xiexiandeite is a high-pressure mineral, forming under conditions of the upper mantle of the Earth. It forms at depths exceeding 125 km. It is transported to the surface as an inclusion in diamond crystals, which in turn are found in ophiolitic rocks (fragments of oceanic crust and mantle) uplifted to the surface by tectonic processes. ## Mineral associations This mineral occurs in paragenesis with diamond, coesite (a high-pressure polymorph of SiO₂), chromite, and native iron. ## Localities The only confirmed locality of xiexiandeite in the world is the Luobusha ophiolite in the Yarlung Zangbo suture zone, in the Tibetan Autonomous Region of China.

Rarity

Extremely rare

For collectors

## Quality criteria These criteria do not apply, as xiexiandeite is not a collectible mineral. Its value is purely scientific and depends on the quality of analytical data that can be obtained from the sample, and on the geological context of the find. ## Popular localities The mineral is known only from its type locality in Tibet, which is not accessible to collectors.

Care and storage

## Cleaning Not applicable. The mineral occurs exclusively as microscopic inclusions in other minerals, mainly diamond. It does not form collectible specimens requiring cleaning. ## What to avoid Not applicable. Any physical or chemical interactions would affect the host mineral in which xiexiandeite is enclosed. ## Storage Specimens containing xiexiandeite are stored in scientific and research collections. They are not available for collectors.

Sources

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