Magnesiohongruiite-(Fe^3+^)

Chemical formula: (Mg<sub>2</sub>Fe<sup>3+</sup>)Fe<sup>3+</sup>Nb<sup>5+</sup>O<sub>7</sub>(OH)

Magnesiohongruiite-(Fe³⁺) is a rare oxide from the hongruiite group, characterized by high magnesium and iron content.

## Characteristics Magnesiohongruiite-(Fe³⁺) is a very rare mineral belonging to the hongruiite group. It occurs as extremely small, tabular or bladed crystals, reaching sizes up to 15x5x2 micrometers. It typically forms aggregates with other minerals. Due to the microscopic size of the crystals, its visual features are difficult to observe without specialized equipment. ## Physical Properties The crystals exhibit a metallic luster. Hardness and density have not been precisely measured due to the small size of the samples; however, the density calculated based on the chemical formula and unit cell parameters is 5.23 g/cm³. The mineral is opaque. ## Colors and Varieties In reflected light, magnesiohongruiite-(Fe³⁺) is gray with a slight brownish tint. Neither pleochroism nor internal reflections have been observed. No varieties of this mineral have been distinguished. ## History and Name The mineral was discovered in the Bayan Obo carbonatite complex in Inner Mongolia, China. Its name honors the Chinese mineralogist Hongrui Qian (born 1938) for his contributions to the study of the Bayan Obo deposits. The "-(Fe³⁺)" suffix in the name indicates the dominance of trivalent iron in a specific position within the crystal structure. It was approved by the International Mineralogical Association (IMA) in 2023 under number IMA2023-076. ## Uses Due to its extreme rarity and microscopic size, magnesiohongruiite-(Fe³⁺) has no commercial or industrial applications. It is solely an object of scientific and collecting interest for specialized institutions.

Properties

Luster
Metallic
Density
5.23
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Identification of magnesiohongruiite-(Fe³⁺) is possible only using advanced analytical techniques, such as scanning electron microscopy (SEM) with chemical composition analysis (EDS) and backscattered electron diffraction (EBSD) or X-ray diffraction (XRD). In reflected light under a polarizing microscope, it is gray with a brownish tint. ## Distinguishing from Similar Minerals This mineral is difficult to distinguish from other niobium and iron oxides, and especially from other minerals of the hongruiite group, such as hongruiite-(Y). Definitive distinction requires precise chemical analysis to determine the dominant elements in specific structural positions. ## Crystal Forms It forms very small, elongated, bladed or tabular crystals that occur as inclusions in other minerals, mainly aegirine.

Geological environment

## Genesis Magnesiohongruiite-(Fe³⁺) forms in hydrothermal processes associated with sodium metasomatism within carbonatites. In the type locality (Bayan Obo), it is a product of the alteration of earlier niobium minerals. ## Mineral Associations This mineral co-occurs with aegirine, barite, fluorite, monazite-(Ce), bastnäsite-(Ce), eschynite-(Nd), and hongruiite-(Y). ## Localities The only confirmed occurrence of magnesiohongruiite-(Fe³⁺) in the world is the Bayan Obo iron, niobium, and rare earth element deposit, located in the Inner Mongolia Autonomous Region in northern China.

Rarity

Extremely rare

For collectors

## Quality Criteria As a microscopic mineral, the collector's value of magnesiohongruiite-(Fe³⁺) is not assessed based on typical criteria such as crystal size or color. The most valuable specimens are those where the presence of the mineral has been analytically confirmed, and its association with other rare minerals is well-documented. The value lies in the scientific significance and rarity of the specimen, not its aesthetics. ## Popular Localities The only source of specimens is the type locality – the Bayan Obo carbonatite complex in China. Material from this location is extremely difficult to obtain.

Care and storage

## Cleaning Due to the microscopic size of the crystals and their occurrence within the host rock, individual cleaning of specimens is not practical. Specimens should be treated as a whole with the rock matrix. ## What to Avoid Avoid contact with strong acids and chemicals that could damage both the mineral itself and co-occurring phases. Also, avoid ultrasonic cleaners and steam cleaning. ## Storage Specimens containing magnesiohongruiite-(Fe³⁺) should be stored under stable conditions, away from dust and moisture, preferably in specialized display boxes or cabinets. They should be protected from mechanical damage.

Sources

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