Neighborite, NaMgF3, occurs in the dolomitic oil shale of the Eocene Green River formation of South Ouray, Uintah County, Utah. It is associated with dolomite and quartz, and the accessory minerals burbankite, nahcolite, barytocalcite, garrelsite, wurtzite, calcite, and pyrite. The mineral occurs in pink rounded grains or in clear twinned octahedral crystals 0.1 to 0.5 mm. in diameter. It has a vitreous luster, uneven fracture, a hardness of 4.5, and is insoluble in water. The specific gravity is 3.03 (measured) and 3.06 (calculated). Neighborite is optically anisotropic with very low birefringence and a mean index of refraction 1.364. The crystals are invariably complexly twinned.
X-ray powder patterns show that neighborite is isostructural with perovskite, CaTiO3. The pattern was indexed on an orthorhombic unit cell having a probable space group Pcmn, with dimensions a = 5.363 Å, b = 7.676 Å and c = 5.503 Å. The expansion of the unit cell was followed to 954° C. using a heating stage on the diffractometer. At 760° C. the orthorhombic a and c axes coalesce and from 760° to 900° C. the cell is tetragonal or pseudo-tetragonal with a=3.942 Å and c = 3.933 Å (at 760° C.). At 900° ±25° C. the unit cell becomes cubic with a=3.955 Å. The crystal structure of neighborite is interpreted in terms of the known structure of perovskite.