Bobdownsite is the fluorine bearing mineral of the whitlockite group of phosphate minerals whose formula is Ca96. It is isotypic with whitlockite and was misidentified as such until proper chemical analysis. Whitlockites structure and relationships with other phosphate compounds has been extensively studied. Bobdownsite was first recovered from Big Fish River, Yukon, Canada from a Lower Cretaceous outcrop of bedded ironstones and shales. Bobdownsite is named after Robert Terrace Downs a professor of mineralogy in the Department of Geosciences at the University of Arizona, who lived and worked in the Yukon Territory in the 1970s. Bobdownsite is unique because it is the first known naturally forming phosphate to contain a P-F bond. Bobdownsite is closely related to whitlockite and merrillite.
Relationship of bobdownsite to other species
Member of the whitlockite group: Other members of group:
Bobdownsite, whitlockite, and merrillite are all important phosphate minerals found closely related in lunar rocks. Merrillite crystals with considerable amounts of fluorine and traces of chlorine were found in several Martian meteorites. But more research conducted on bobdownsite suggests that an F content of greater that about 0.9 wt%, including F- bearing merrillite, is bobdownsite.
In Yukon, Canada bobdownsite is found composed of anhedral and euhedral crystals, tabular, colorless in transmitted light, transparent, with a white streak and a vitreous luster. The Tip Top mine sample has druses of pale purple, yellow, or transparent colorless rhombohedral crystals, brittle, with a hardness about 5, no cleavage, parting or twinning, and an uneven and subconchoidal fracture. Bobdownsite has a density of 3.14 and 3.16 g/cm3 and is insoluble in water, acetone, or hydrochloric acid.
Crystallography
Bobdownsite is isostructural with whitlockite having a trigonal crystal system and R3c space group. The point group is 3m with unit cell parameters ɑ 1.3224 Å, c 37.70 Å, V 3420.7 Å3. Bobdownsite has trigonal - ditrigonal pyramid tabular, euhedral crystals. The mineral is characterized by −16 ligands held together by intralayer Ca2+ cations to form layers parallel to. The layers parallel to are further linked by intralayer Ca2+, Na+, and Sr2+ cations along . These ligands can be referred to as the "Mg pinwheel". The pinwheels are formed in closest packed planar arrangement using the term cubic eutaxy. For this arrangement to happen there are tetrahedral and octahedral sites. The octahedral sites contain PO3F tetrahedral groups which are approximately in the midplane between the two pinwheel layers and are each coordinated to the closest two interlayer Ca cations by the oxygen. The P position in the PO3F is located either just above or just below the midplane between the two pinwheel layers, leading to the F to point either along +c or -c. The metal of the whitlockite- type compounds is slightly offset from the origin along c and is octahedrally coordinated residing at the center of the M pinwheel. The layers stack in an ABAB fashion along the c, there is no obvious eutaxy, they can be described as chains of corner-sharing LiO4 tetrahedra cross connected by PO3F tetrahedra.
