A reexamination of the turquoise group: the mineral aheylite, planerite (redefined), turquoise and coeruleolactite

Abstract

AbstractThe turquoise group has the general formula: A0–1B6(PO4)4−x(PO3OH)x(OH)8.4H2O, where x = 0–2, and consists of six members: planerite, turquoise, faustite, aheylite, chalcosiderite and an unnamed Fe2+-Fe3+ analogue. The existence of ‘coeruleolactite’ is doubtful. Planerite is revalidated as a species and is characterized by a dominant A-site vacancy. Aheylite is established as a new member of the group, and is characterized by having Fe2+ dominant in the A-site.Chemical analyses of 15 pure samples of microcrystalline planerite, turquoise, and aheylite show that a maximum of two of the (PO4) groups are protonated (PO3OH) in planerite. Complete solid solution exists between planerite and turquoise. Other members of the group show variable A-site vacancy as well. Most samples of ‘turquoise’ are cation-deficient or are planerite. Direct determination of water indicates that there are 4 molecules of water.Planerite, ideally ☐Al6(PO4)2(PO3OH)2(OH)8.4H2O, is white, pale blue or pale green, and occurs as mamillary, botryoidal crusts as much as several mm thick; may also be massive; microcrystalline, crystals typically 2–4 micrometres, luster chalky to earthy, H. 5, somewhat brittle, no cleavage observed, splintery fracture, Dm 2.68(2), Dc 2.71, not magnetic, not fluorescent, mean RI about 1.60. a 7.505(2), b 9.723(3), c 7.814(2) Å, α 111.43°, β 115.56°, γ 68.69°, V 464.2(1) Å3, Z = 1.Aheylite, ideally Fe2+Al6(PO4)4(OH)8.4H2O, is pale blue or green, and occurs as isolated and aggregate clumps of hemispherical or spherical, radiating to interlocked masses of crystals that average 3 micrometres in maximum dimension; porcelaneous-subvitreous luster, moderate to brittle tenacity, no cleavage observed, hackly to splintery fracture, not magnetic, not fluorescent, biax. (+), mean RI is about 1.63, Dm 2.84(2), Dc 2.90. a 7.400(1), b 9.896(1), c 7.627(1) Å, α 110.87°, β 115.00°, γ 69.96°, V 460.62(9) Å3, Z = 1.