Bobdownsite

Bobdownsite is the fluorine-bearing mineral of the whitlockite group of phosphate minerals whose formula is Ca9(Mg)(PO4)6(PO3F). 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
General
CategoryPhosphate minerals
Formula
(repeating unit)		Ca9(Mg)(PO4)6(PO3F)
Strunz classification8.AC.45
Dana classification38.3.4.8
Crystal systemTrigonal
Crystal classDitrigonal pyramidal (3m)
(same H-M symbol)
Space groupR3c
Unit cella = 10.3224 Å,
c = 37.070 Å; Z = 6
Identification
Formula mass1,054.36 g/mol
ColorColorless, gray-white, light pink, light yellow
Crystal habittabular, euhedral crystals
CleavageNone
Fractureuneven and subconchoidal
TenacityBrittle
Mohs scale hardness5
LusterVitreous
StreakWhite
DiaphaneityTransparent
Specific gravity3.14 to 3.16
Optical propertiesuniaxial (-)
Refractive indexnω = 1.625(2) nε = 1.622(2)
Ultraviolet fluorescencenone
Diagnostic featuresunit cell dimension
References[1][2][3]

Bobdownsite is closely related to whitlockite and merrillite.

Relationship of bobdownsite to other species

Member of the whitlockite group:[2]

Other members of group:

  • Ferromerrillite: Ca9NaFe2+(PO4)7
  • Hedegaardite: (Ca,Na)9(Ca,Na)Mg(PO4)6(PO3OH)
  • Merrillite: Ca9NaMg(PO4)7
  • Strontiowhitlockite: Sr9Mg(PO4)6(HPO4)
  • Tuite: Ca3(PO4)2
  • Whitlockite: Ca9Mg(PO4)6(HPO4)

Geological occurrences

Bobdownsite is originally found from Big Fish River, Yukon, Canada (about Latitude 68°28'N, Longitude 136°30'W). Bobdownsite occurs in a vein from an east- west- trending faulted vein. Samples recovered from this exposure are found with Lower Cretaceous bedded ironstones and shales. Bobdownsite is a phosphate mineral with its exposure downstream from phosphate nodule slopes. The vein at the exposure is composed of anhedral and euhedral crystals and is 0 to 4 cm wide. It occurs associated with siderite, lazulite, kulanite, gormanite, quartz and collinsite. Bobdownsite has also been reported from the Tip Top mine of Custer County, South Dakota. Furthermore, it has been found with other witlockite group minerals in Martian meteorites.[4]

Meteorites

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.[5] But more research conducted on bobdownsite suggests that an F content of greater that about 0.9 wt%, including F- bearing merrillite, is bobdownsite.

Physical properties

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.[4]

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(3) Å, c 37.70(2) Å, V 3420.7(6) Å3. Bobdownsite has trigonal - ditrigonal pyramid[3] tabular, euhedral crystals. The mineral is characterized by [M(PO4)6]−16 ligands held together by intralayer Ca2+ cations to form layers parallel to (001). The layers parallel to (001) are further linked by intralayer Ca2+, Na+, and Sr2+ cations along [001]. 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. [4]

See also

References
  1. Mineralienatlas
  2. "Bobdownsite: Bobdownsite mineral information and data". www.mindat.org. Retrieved 2015-12-06.
  3. "Bobdownsite Mineral Data". webmineral.com. Retrieved 2015-12-06.
  4. Tait, Kimberly T.,Madison C. Barkley, Richard M. Thompson, Marcus J. Origlieri, Stanley H. Evans, Charles T. Prewitt, and Hexiong Yang, "Bobdownsite, A New Mineral Species From Big FIsh River, Yukon, Canada, and Its Structural Relationship With Whitlockite-Type Compounds", The Canadian Mineralogist, 49.4 (2011): (1065-1078). Print.
  5. Gnos, E., et al. (2002): Sayh al Uhaymir 094: a new martian meteorite from the Oman desert: Meteoritics & Planetary Science 37, 835-854.
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