Neighborite

Neighborite
General
Category	Halide
Formula (repeating unit)	NaMgF3
IMA symbol	Nbo
Strunz classification	3/C.03-70
Crystal system	Orthorhombic
Crystal class	mmm(2/m 2/m 2/m)
Space group	Pcmn
Unit cell	a = 5.363 Å, b = 7.676 Å c = 5.503 Å; Z = 4
Identification
Color	Colorless, mahogany, brown
Crystal habit	Subangular rounds or octahedra
Twinning	Interpenatration
Fracture	Uneven
Mohs scale hardness	4.5
Luster	Vitreous
Diaphaneity	Transparent to opaque
Specific gravity	3.03
Density	3.03 g/cm3 Measured; 3.06 g/cm3 Calculated
Optical properties	Anisotropic
Refractive index	1.364±0.002
Birefringence	δ=0.003
Dispersion	r < v weak
References

Neighborite is a mineral composed of a double fluoride salt of sodium and magnesium. It was initially recognized as pink and brown masses of round grains in the Green River Formation South of Ouray, Utah. Originally found in dark dolomitic oil shales, neighborite typically forms as subrounded or octahedral crystals. Neighborite received its name from Frank Neighbor, a geologist for the Sun Oil Co. based in Salt Lake City, Utah. Due to Mr. Neighbor's work in discovering neighborite, along with other newly discovered minerals to the Green River Formation, the mineral was named after him. Neighborite is in the family of perovskite crystal structure minerals, possessing a general formula of ABX₃.

Occurrence

Neighborite is found in association with minerals such as burbankite, nahcolite, wurtzite, barytocalcite, garrelsite, pyrite, calcite, and quartz. The mineral most commonly occurs on top of dark colored dolomitic oil shales. The resulting grains are subrounded in shape. Some neighborite crystals form as inclusions in barytocalcite. Octahedral crystals of neighborite exist in nature, but is generally uncommon relative to the subangular crystals. Neighborite is a rare mineral, only being found in select localities around the world. Regions where neighborite is currently found include select wells in or near Ouray, Utah; Montreal, Quebec; Oslo, Norway; Murmansk Oblast, Russia; Arusha, Tanzania.

Physical properties

Neighborite is a colorless or pink to brown colored mineral exhibiting a vitreous luster. It exhibits a hardness of 4.5 on the Mohs hardness scale. It does not portray any specific cleavage planes, but instead exhibits an uneven fracture. Neighborite is insoluble in distilled water, but dissolves at a quicker rate with increased acidic conditions.

Optical properties

Neighborite is an anisotropic mineral, meaning it has more than one refractive index. However, the birefringence of the mineral is rather low, at 0.003. The measured refractive indexes are 1.364±0.002. The octahedral {111} crystals of neighborite occasionally exhibit twinning. {100} cubic faces also exist as truncations.

Chemical properties

Neighborite belongs to the perovskite family of minerals. These minerals have a general formula of ABX₃. The structure consists of B atoms in an octahedron, with A and X atoms forming the cubic and octahedral cells of the structure, respectively. Perovskites are most notably used as solar cells, achieving a general efficiency of around 25%.

Chemical composition

Element	wt%
Na	22.05
Mg	23.31
F	54.64
Total	100.00

X-ray crystallography

Neighborite is an orthorhombic mineral, containing the space group Pcmn. The unit cell dimensions are a=5.363 Å, b=7.676 Å, c=5.503 Å, α=90.00°, β=90.00°, γ=90.00°. The dimensions of neighborite are extremely similar to those of a basic perovskite structure. It is therefore considered part of the perovskite supergroup of minerals. As higher temperatures are achieved, the three axes lengths of neighborite get closer together. At 900°C, the axes lengths are equal, giving neighborite a cubic cell. However, this cubic structure cannot be quenched, and can only exist at the high temperature range.

See also

• List of minerals