Coconinoite
Coconinoite on a sandstone matrix
General
Category	Phosphate minerals
Formula (repeating unit)	Fe3+2Al2(UO2)2(PO4)4(SO4)(OH)2•20(H2O)
IMA symbol	Coc[1]
Strunz classification	8.EB.35
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 12.45(6) Å, b = 12.96(3) Å, c = 17.22(5) Å; β = 105.7°; Z = 4
Identification
Formula mass	489.01 gram/mol
Color	Pale creamy yellow
Crystal habit	As lathlike to platy grains, in microcrystalline aggregates seams and crusts.
Mohs scale hardness	1-2
Luster	Adamantine - pearly
Streak	White
Diaphaneity	Translucent
Specific gravity	2.70
Optical properties	Biaxial (-)
Refractive index	nα = 1.550 nβ = 1.588 nγ = 1.590
Birefringence	δ = 0.040
Pleochroism	X = colorless; Y = Z = pale yellow. Orientation: Y = elongation of laths with positive elongation; Y at 8°-25° to elongation of laths with negative elongation.
2V angle	Measured: 28° to 43°, Calculated: 24°
Other characteristics	Radioactive
References	[2][3][4]

Coconinoite is a uranium ore that was discovered in Coconino County, Arizona. It is a phosphate mineral; or uranyl phosphate mineral along with other subclass uranium U⁶⁺ minerals like blatonite, boltwoodite, metazeunerite and rutherfordine.

Composition

The chemical formula is Fe₂Al₂(UO₂)₂(PO₄)₄(SO₄)(OH)₂·20H₂O.^[5] The chemical formula was derived from the spectrographic analysis.^[5]

Physical properties

The mineral has a white streak and a pale creamy yellow color. The mineral occurs as microscopic crystals, the largest found is 6 by 20 micrometers. It is a radioactive mineral, but not fluorescent.^[5] Upon heating for dehydration it is found that the mineral loses some of its SO₂ at 600 to 800 °C.

Geologic occurrence

It occurs in the oxidized zone of vanadium-poor Colorado Plateau-type uranium deposits of Utah and Arizona. It occurs in association with gypsum, jarosite, limonite, quartz, clay minerals and coalized wood at the Jomac mine, Utah.^[4]

Coconinite was first described in 1966 for occurrences in the Huskon Mines, Cameron, Cameron District and the Sun Valley Mine, Vermillion Cliffs District, Coconino County, Arizona. It was named for Coconino County.^[3]

References