A Reexamination of the Turquoise Group: The Mineral Aheylite, Planerite (Redefined), Turquoise and Coeruleolactite

Eugene E. Foord and Joseph E. Taggart Jr.
M.S. 905, U.S. Geological Survey, Box 25046 Denver Federal Center, Denver, CO 80225, USA
M.S. 973, U.S. Geological Survey, Box 25046 Denver Federal Center, Denver, CO 80225, USA

Abstract: The 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 □A16(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.

Keywords: turquoise group • planerite • aheylite • ‘coeruleolactite’ • X-ray diffraction data

Mineralogical Magazine; February 1998 v. 62; no. 1; p. 93-111; DOI: 10.1180/002646198547495
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)