Stanfieldite

Chemical formula: Ca₄Mg₅(PO₄)₆

Stanfieldite is a rare calcium and magnesium phosphate, found mainly in pallasite meteorites, forming small grains with a resinous luster.

## Characteristics Stanfieldite is a very rare mineral from the phosphate group, occurring almost exclusively in meteorites. It forms small, anhedral (irregularly shaped) grains or aggregates, usually less than 1 mm in size. Its presence is often associated with other phosphates within a metallic iron-nickel matrix or surrounding olivine crystals. ## Physical Properties This mineral is characterized by a hardness of approximately 4 on the Mohs scale. It has a luster described as resinous to vitreous and is typically translucent. The density of stanfieldite is about 3.16 g/cm³. ## Colors and Varieties Stanfieldite most often ranges in color from light yellow, through yellowish-brown, to reddish-brown. No color varieties or commercial varieties are distinguished. ## History and Name The mineral is named in honor of Charles A. Stanfield, a meteorite collector from Arkansas, USA, who provided a meteorite sample for study in which this mineral was first identified. It was described and approved as a new mineral species in 1967 by Louis H. Fuchs. The type locality is the Estherville pallasite meteorite, found in Emmet County, Iowa, USA. ## Applications Stanfieldite has no industrial applications. Its significance is purely scientific, as a component of meteorites providing information about the formation processes of celestial bodies, and collectible, although due to its small size and rarity, it is primarily of interest to specialized collectors of meteoritic minerals.

Properties

Mohs hardness
4.5-5
Color
Amber to pale red
Luster
Resinous
Streak
White
Density
3.15
Cleavage
Imperfect on {100}
Fracture
Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification Identifying stanfieldite is extremely difficult without advanced analytical methods. Within a pallasite meteorite, it can be suspected as fine, yellowish or brownish grains with a resinous luster, often in close proximity to other phosphates such as farringtonite or merrillite. Identification relies mainly on chemical analysis (EDS) and X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other phosphates found in meteorites, such as merrillite, whitlockite, or farringtonite, which often have a similar appearance and habit. Definitive distinction is possible only through laboratory methods. ## Crystal Forms Stanfieldite forms irregular, anhedral grains and aggregates. It is not observed in the form of well-developed, euhedral crystals.

Geological environment

## Genesis Stanfieldite is a mineral of extraterrestrial origin. It forms in meteorites, mainly pallasites, which consist of olivine crystals embedded in a metallic iron-nickel matrix. It is believed to crystallize in the late stages of pallasite parent body formation, as a result of reactions involving phosphorus present in the metallic alloy. ## Mineral Associations It most commonly co-occurs with olivine, kamacite, taenite, schreibersite, troilite, as well as other rare phosphates such as farringtonite and merrillite. ## Localities Its occurrence has been confirmed in many pallasites. The most important include the meteorites: Estherville (Iowa, USA - type locality), Imilac (Atacama Desert, Chile), Brahin (Belarus), Brenham (Kansas, USA), Esquel (Argentina), and Fukang (China).

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of stanfieldite is inextricably linked to the quality of the meteorite specimen in which it occurs. Since the grains are microscopic or very small, their individual assessment is rarely possible. The value of a specimen is enhanced by the abundance of stanfieldite aggregates, their more intense color (e.g., reddish-brown), and well-documented provenance from a specific, known meteorite fall. The most prized are polished pallasite slices where stanfieldite grains are visible surrounded by olivines and metal. ## Popular Localities Specimens from classic pallasite localities such as Imilac, Esquel, or Fukang are most sought after by collectors due to the beauty and stability of these meteorites.

Care and storage

## Cleaning Specimens containing stanfieldite, due to their meteoritic origin and the presence of metallic iron, should generally not be wet cleaned. Dust can be removed very carefully using a soft brush or compressed air from a safe distance. ## What to Avoid Contact with water, chemicals, and cleaning agents must be strictly avoided, as they can cause corrosion of the surrounding iron-nickel matrix. The mineral is sensitive to acids. It should be protected from moisture and sudden temperature changes. ## Storage Meteorite specimens containing stanfieldite should be stored in a dry environment, preferably in a sealed container (e.g., a "membrane box") or in a display case with a desiccant (e.g., silica gel). This reduces the risk of oxidation of the metallic components of the meteorite.

External references

Sources

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