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

U. S. Geological Survey, Denver, Co, United States

The turquoise group has the general formula: A (sub 0-1) B ₆ (PO ₄ ) (sub 4-x) (PO ₃ OH) _x (OH) ₈ .4H ₂ O, where x = 0-2, and consists of six members: planerite, turquoise, faustite, aheylite, chalcosiderite and an unnamed Fe (super 2+) -Fe (super 3+) 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 Fe (super 2+) 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 (PO ₄ ) groups are protonated (PO ₃ OH) 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 []Al ₆ (PO ₄ ) ₂ (PO ₃ OH) ₂ (OH) ₈ .4H ₂ O, 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, D _m 2.68(2), D _c 2.71, not magnetic, not fluorescent, mean RI about 1.60. a 7.505(2), b 9.723(3), c 7.814(2) Aa, alpha 111.43 degrees , beta 115.56 degrees , Gamma 68.69 degrees , V 464.2(1) Aa ³ , Z = 1. Aheylite, ideally Fe (super 2+) Al ₆ (PO ₄ ) ₄ (OH) ₈ .4H ₂ O, 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, D _m 2.84(2), D _c 2.90. a 7.400(1), b 9.896(1), c 7.627(1) Aa, alpha 110.87 degrees , beta 115.00 degrees , gamma 69.96 degrees , V 460.62(9) Aa ³ , Z = 1.

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